Semiconductor integrated circuit device and a method of manufacturing the same

ABSTRACT

A semiconductor device including a memory cell featuring a first gate insulating film over a semiconductor substrate, a control gate electrode over the first gate insulating film, a second gate insulating film over the substrate and a side wall of the control gate electrode, a memory gate electrode over the second gate insulating film arranged adjacent with the control gate electrode through the second gate insulating film, first and second semiconductor regions in the substrate positioned on a control gate electrode side and a memory gate side, respectively, the second gate insulating film featuring a first film over the substrate, a charge storage film over the first film and a third film over the second film, the first film having a first portion between the substrate and memory gate electrode and a thickness greater than that of a second portion between the control gate electrode and the memory gate electrode.

This application is a Continuation of U.S. application Ser. No.15/358,950, filed Nov. 22, 2016, which is a Continuation of U.S.application Ser. No. 15/079,702, filed Mar. 24, 2016, which is aContinuation of U.S. application Ser. No. 14/809,808, filed Jul. 27,2015, which, in turn, is a Continuation of U.S. application Ser. No.14/560,447, filed Dec. 4, 2014, which, in turn, is a Continuation ofU.S. application Ser. No. 14/217,748, filed Mar. 18, 2014; which, inturn, is a continuation of U.S. application Ser. No. 13/955,943, filedJul. 31, 2013 (now U.S. Pat. No. 8,674,432), which, in turn, is acontinuation of U.S. application Ser. No. 13/531,802, filed Jun. 25,2012 (now U.S. Pat. No. 8,507,975), which, in turn, is a Continuation ofU.S. patent application Ser. No. 13/097,731, filed Apr. 29, 2011 (nowU.S. Pat. No. 8,222,686), which, in turn, is a Continuation of U.S.patent application Ser. No. 12/868,990, filed Aug. 26, 2010 (now U.S.Pat. No. 7,952,135), which, in turn, is a Continuation of U.S. patentapplication Ser. No. 12/473,613, filed May 28, 2009 (now U.S. Pat. No.7,807,530), which, in turn, is a Continuation of U.S. patent applicationSer. No. 12/325,054, filed Nov. 28, 2008 (now U.S. Pat. No. 7,544,988),which, in turn, is a Continuation of U.S. patent application Ser. No.11/212,707, filed Aug. 29, 2005 (now U.S. Pat. No. 7,525,145), which, inturn, is a Continuation of U.S. patent application Ser. No. 10/400,469,filed Mar. 28, 2003 (now U.S. Pat. No. 7,045,848), and the entiredisclosures of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates in general to a semiconductor integratedcircuit device having a nonvolatile memory cell transistor (nonvolatilememory element), and to a method of manufacture thereof; and, moreparticularly, the invention relates, for example, to a technology thatis effective when applied to a semiconductor integrated circuit deviceequipped with an on-chip nonvolatile memory, using a nonconductivecharge trap film in an information retention region together with a CPU(Central Processing Unit).

BACKGROUND OF THE INVENTION

As a memory device for storing data or program-configured data,attention has recently been focused on a flash EEPROM (hereinaftercalled a “flash memory”), which is defined as a nonvolatile memorydevice that is capable of electrically erasing data that is stored inpredetermined units in batch form and of electrically writing data. In aflash memory, each memory cell is made up of an electrically erasableand programmable nonvolatile memory element. The flash memory is capableof erasing data that is temporarily written into a corresponding memorycell or program-configured data and of rewriting (programming) new dataor program-configured data into the corresponding memory cell.

The storage of an electrical charge in a flash memory has heretoforebeen performed by storing or accumulating electrons in a floating gatecomprising a polysilicon film and which is electrically isolated fromthe surroundings. Such a conventional memory cell has been called a“floating gate type flash”. The injection of hot electrons has generallybeen used as such an electron storage operation, i.e., a so-called writeoperation. The operation of discharging accumulated electrons out of thefloating gate has been performed by a tunneling current which passesthrough a gate oxide film. When writing and erasure are repeated, acharge trap is formed inside the gate oxide film, and the surface levelor state density increases at an interface between the substrate and thegate oxide film. In particular, the former has an essential problem inthat the charge retention characteristic, i.e., the post-writingretention characteristic, is degraded.

As a method of resolving such a problem, a memory cell system hasrecently been proposed which makes use of a nonconductive charge trapfilm for the purpose of charge storage of the EEPROM. This has beendisclosed in, for example, U.S. Pat. No. 5,768,192, U.S. Pat. No.5,966,603, U.S. Pat. No. 6,011,725, U.S. Pat. No. 6,180,538, and B.Eitan et al., “Can NROM, a 2-bit, Trapping Storage NVM Cell, Give a RealChallenge to Floating Gate Cell”, International Conference on SolidState Devices and Materials, Tokyo, 1999.

U.S. Pat. No. 5,768,192 has disclosed, for example, a system wherein, asshown in FIG. 58, a silicon nitride film 183 is interposed betweeninsulating films 182 and 184, such as a silicon oxide film, etc.,whereby a so-called laminated film having an ONO (Oxide/Nitride/Oxide)structure is used as a gate insulating film. In this first conventionalmemory cell 0V is applied to a source 187, 5V is applied to a drain 186and 9V is applied to a control gate 185, so as to turn on thetransistor, thereby injecting hot electrons developed in theneighborhood of the drain 186 and trapping them into the silicon nitridefilm 183, whereby writing is performed.

As compared with a system for effecting charge storage on a polysiliconfilm corresponding to a continuous conductive film, a charge storagesystem such as provided by the conventional first memory cell ischaracterized in that, since the trapping of electrons into the siliconnitride film 183 is noncontiguous and discrete, all of the storedcharges do not disappear, even where charge leakage passes, such as viapin holes or the like that occur in part of the oxide film 182, and theretention characteristic is essentially strong. An erase operation ofsuch a memory cell is performed by, as shown in FIG. 59, applying 3V, 5Vand −3V to the source 187, drain 186 and control gate 185, respectively,to forcibly reverse the neighborhood of the drain 186 on the siliconsurface side and inject hot holes, that are generated by a band-to-bandtunnel phenomenon caused by an energy band that is significantlydeformed by a strong electric field, into the silicon nitride film 183,to thereby neutralize the already-trapped electrons.

U.S. Pat. No. 5,408,115 and U.S. Pat. No. 5,969,383, respectively, havedisclosed a memory cell system which has a split gate using sidespacers, in which charge storage is effected on an ONO film serving as amemory cell structure. A write/erase system embodying this technique isshown in FIGS. 60 and 61. In this conventional second memory cell, asshown in FIG. 60, a select gate 163 is disposed on a gate oxide film 162which is formed on the surface of a substrate 161, and lower oxide films165, silicon nitride films 166 and upper oxide films 167 are laminatedat a peripheral portion of the select gate 163, followed by provision ofside spacer-shaped control gates 168. Since the source 164 of thisconventional second memory cell is formed immediately after theprocessing of the select gate 163, and the drain 169 is formed after theprocessing of the control gates 168, only the control gate 168 on thedrain 169 side functions as a gate electrode.

A write operation for the conventional second memory cell, as shown inFIG. 60, is performed by applying 5V, 1V and 10V to the correspondingdrain 169, select gate 163 and control gate 168, respectively, to turnon a channel and accelerate electrons travelling from the source 165within a strong lateral electric field produced in a channel regiondisposed below the boundary between the select gate 163 and the controlgate 168, so as to bring them to a hot electron state, and the hotelectrons are caused to pass through the lower oxide film 165, fromwhich they are injected and trapped into the silicon nitride film 167.Since the injection positions of the hot electrons are not located inthe neighborhood of the drain, this operation is generally called a“source side injection (SSI) system”. An erase operation for thisconventional second memory cell, as shown in FIG. 61, is performed byapplying 14V to only the corresponding control gate 168, to thereby drawelectrons that are trapped into the corresponding silicon nitride film166 into the control gate 168 as a tunneling current flowing into theupper oxide film 167. Since the injection of electrons from thesubstrate 161 also occurs due to a tunneling current flowing via thelower oxide film 165 during the erase operation, there is a need to formthe lower oxide film 165 to that it is thicker than the upper oxide film167.

Further, in a read operation for the conventional second memory cell, asshown in FIG. 62, 2V and 5V are respectively applied to thecorresponding drain 169 and select gate 163 so as to turn on thechannel, and 2V is applied to the corresponding control gate 168, tothereby determine the high or low level of a threshold voltage, based onthe presence or absence of the electrons trapped into the siliconnitride film, from the magnitude of the drain current. As compared withthe conventional first memory cell of FIGS. 58 and 59, the conventionalsecond memory cell of FIGS. 60-62 has the advantage of reducing thedrain current necessary for the write operation and achieving areduction in power. This is because, since the conventional secondmemory cell is provided with the select gate 163, the channel current atthe time of writing can be controlled so that it is low. The channelcurrent can be reduced to 1/100 or less of that for the conventionalfirst memory cell.

Furthermore, U.S. Pat. No. 5,408,115 has disclosed a conventional thirdmemory cell, whose structure is shown in FIG. 63. The conventional thirdmemory cell has a structure in which the structural positions of theselect and control gates employed in the conventional second memory cellare changed. In the conventional third memory cell, a control gate 175is formed above a lamination consisting of a lower oxide film 172, asilicon nitride film 173 and an upper oxide film 174, and, thereafter,gate oxide films 177 and side spacer-shaped select gates 178 are formed.The voltages that are set to effect write, erase and read operations ofthe present conventional third memory cell are similar to those of theconventional second memory cell.

A conventional fourth memory cell system, whose sectional views areshown in FIGS. 64 and 65, has been disclosed in I. Fujiwara, et al.,“High Speed program/erase sub 100 nm MONOS memory”, NonvolatileSemiconductor Memory Workshop, August, 2001, p 75. As shown in FIG. 64,an ONO (Oxide/Nitride/Oxide) laminated film, comprising a siliconnitride film 193 interposed between insulating films 192 and 194, suchas silicon oxide films, etc., is formed as a gate insulating film, and12V is applied to a control gate 195 to inject electrons from thesemiconductor substrate 191 side by a tunneling current and trap theminto the silicon nitride film 193, thereby performing an erase operationthat is brought into a high threshold voltage state. 6V is applied to asource 197 and a drain 196, and −6V is applied to the control gate 195to forcibly reverse a silicon surface near a source/drain and inject hotholes, that are developed by a band-to-band tunneling phenomenon causedby an energy band that is greatly deformed by a strong field into thesilicon nitride film 193, so as to neutralize the already-trappedelectrons, thereby performing a write operation that is brought into alow threshold voltage state.

SUMMARY OF THE INVENTION

The present inventors have discovered the following problems as a resultof investigations of the conventional memory cell systems.

A first problem resides in the fact that the drain current at the timeof a reading operation in a low threshold voltage state is small. Thisproblem results in a large drawback in a logic-mixed flash memory modulewhich needs high-speed reading at about 100 MHz, for example. In theconventional first memory cell described in B. Eitan et al., “Can NROM,a 2-bit, Trapping Storage NVM cell, Give a Real Challenge to FloatingGate Cell”, International Conference on Solid State Devices andMaterials, Tokyo, 1999, the insulating films 182 and 184, such assilicon oxide films, etc., as shown in FIGS. 58 and 59, are respectivelyset to 5 nm, and the silicon nitride film 183 is set to 10 nm.Therefore, an oxide film-converted electrical effective thicknessresults in about 15 nm. This value becomes as thick as 1.5 times ascompared with the conventional floating gate type memory cell, whosegate oxide film is designed to be about 10 nm. As compared with a memorycell having the same effective channel width/effective channel length,the read drain current is reduced to about 1/1.5.

In the conventional second memory cell shown in FIGS. 60 through 62, thegate oxide film 163 below the select gate 162 can be designedindependently without depending on the write/erase characteristics ofthe memory cell. The gate oxide film 163 can be designed to be about 5nm, for example. Since an effective channel length can be adjusted bythe side spacer length, even where portions immediately below thecontrol gates 168, and the upper oxide films 165 and 167, arerespectively designed to be 5 nm, the silicon nitride films 166 aredesigned to be 10 nm, and the effective thickness is designed to be 15nm, the effective channel length can be designed so that it is shorterthan the select gate length defined by the minimum processing size. As aresult, the effective channel length of the conventional second memorycell results in a serial length of both the select gate 163 and thecontrol gates 168. However, the read current in a low threshold voltagestate can be designed to be larger than that of the conventional firstmemory cell. From this point of view, the gate electrode to becontrolled has an increased read current, but the conventional secondmemory cell is superior to the first memory cell.

A second problem relates to the reliability of the conventional secondmemory cell. The write/erase operation depends on the source/sideinjection writing of the hot electrons, and the emission and erasure oftunnel electrons effected on the control gate side, as described above.The present inventors have obtained a result in which, as a result ofexecution of a rewrite test by this operation system, the erase timeinterval is significantly degraded when the number of rewritings exceeds10,000 times. As a result of analysis of the cause of this problem, itappears that this has happened because it is difficult for electronstrapped into a corner portion of the silicon nitride film 166,corresponding to an electron trap film disposed in an L shape, to beemitted toward the control gate 168 side. While the amount of electronstrapped into the corner portion of the silicon nitride film 166gradually increases with repetition of the rewrite operation, theeffective thickness of the silicon nitride film 166, as viewed from thecontrol gate 168, is √{square root over ( )}2 times (about 1.4 times)that of a flat portion. Therefore, a reduction in in-film fieldintensity was considered to be a cause of this problem.

When the gate oxide film 163 shown in FIG. 60 is set to 4 nm or less inorder to increase the read current in the low threshold voltage state,it also turned out that a dielectric breakdown failure in the gate oxidefilm 163 occurred during a write operation. This is because, since 10Vis applied to the corresponding control gate 168 so that a channel isformed directly below the control gate 168 during the write operation,as described above, 5V applied to the drain 169 is transferred to thegate oxide film 162 at the end on the control gate side, of the selectgate 163. At this time, the maximum voltage applied to the gate oxidefilm 162 results in (voltage at the drain 169=5V)−(voltage at the selectgate 163=1V)=4V. Thus, the conventional second memory cell has adrawback in that the thickness of the gate oxide film 163 has a lowerlimit, and, thereby, the read current is restricted. In a logic-mixedflash memory module, the thickness of the gate oxide film 163 maypreferably be designed to be the same as the thickness of a gate oxidefilm of a power voltage transistor in terms of simplification of themanufacturing process. While, for example, the thickness of a gate oxidefilm of a logic transistor in a 0.13-μm technology generation rangesfrom 2.5 nm to 3.0 nm, the sharing of the thickness of the gate oxidefilm was difficult from the viewpoint of the withstand voltage of thegate oxide film in the conventional second memory cell.

A third problem relates to the reliability of the conventional fourthmemory cell. The write operation thereof depends on the injection of hotholes from the source/drain junction, as described above. Since thelateral attainable distance of each hot hole produced only in theneighborhood of the source/drain junction in the silicon nitride film193 is about 50 nm, it is necessary to design the effective channellength of the present conventional fourth memory cell so as to be 100 nmor less. Therefore, a problem arises in that a single channel effect isremarkable; stable control on an initial threshold voltage is difficult;and a leakage current, so-called off-leak current on a bit line at thetime that a NOR-type array connection is made, increases and itsvariations become large.

A fourth problem is as follows. Since the gate electrodes for performingthe write/erase operations and the gate electrode for performing theread operation are identical as shown in FIGS. 58, 59, 60 through 62,63, 64 and 65 in the conventional memory cells, the conventional fourthmemory cell shown in FIGS. 64 and 65, for example, has a problem inthat, due to the application of a weak electric field to the insulatingfilm 192 by the application of a power voltage to the control gate 195at the time of the read operation, a weak injection of hot electronsoccurs from the low threshold voltage state in which holes are trappedinto the silicon nitride film 193, so that the threshold voltagegradually increases, and the so-called read disturb life becomes short.As a result, when reading has been performed continuously for ten years,the threshold voltage is increased to greater than the power voltageapplied to the control gate 195, so that an inversion failure in dataoccurs.

An object of the present invention is to provide a technology that iscapable of reading memory information at high speed from a nonvolatilememory cell transistor formed in a semiconductor integrated circuitdevice.

Another object of the present invention is to reduce the parasiticresistance value of a channel portion of a nonvolatile memory celltransistor formed in a semiconductor integrated circuit device.

A further object of the present invention is to provide a semiconductorintegrated circuit device that is capable of preventing an electricalcharge of one polarity from being constantly trapped into a nonvolatilememory cell transistor formed in a semiconductor integrated circuitdevice, and to a method of manufacture thereof.

Yet another object of the present invention is to prevent degradation ofdata retention characteristics due to undesired leakage of electricalcharges stored in a nonvolatile memory cell transistor formed in asemiconductor integrated circuit device.

A still further object of the present invention is to eliminate a highwithstand voltage MIS transistor, which impairs a quick response and islarge in thickness, from a signal path for reading memory informationfrom a nonvolatile memory cell transistor formed in a semiconductorintegrated circuit device.

The above, other objects and novel features of the present inventionwill become apparent from the description provided in the presentspecification and from the accompanying drawings.

Summaries of typical or representative Aspects and features of thepresent invention as disclosed in the present application will bedescribed in brief as follows:

[1] <<Sprit Gate/Antipolarity Charge Injection/Negative SubstratePotential>>

A semiconductor integrated circuit device according to the presentinvention has a memory cell transistor and an access circuit thereforboth provided on a semiconductor substrate. The memory cell transistorincludes, in a first well region of the semiconductor substrate, a pairof memory electrodes, one of which serves as a source electrode and theother serves as a drain electrode, and a channel region interposedbetween the pair of memory electrodes, and includes, on the channelregion, a first gate electrode (3, 123) disposed near the memoryelectrodes with an insulating film (2, 122) interposed between the firstgate electrode (3, 123) and the channel region, and a second gateelectrode (8, 127) disposed on the channel region with an insulatingfilm (4 and 7, 124 and 126) and a charge storage region (6, 125)interposed between the second gate electrode and the channel region andelectrically isolated from the first gate electrode. The access circuitis capable of selecting a first state in which a first negative voltageis applied to the first well region to thereby form a reverse-directionvoltage applied state between the second gate electrode and the memoryelectrode near the second gate electrode and to form an electric fieldfor directing a first polarity charge from the well region side to thecharge storage region. The access circuit is capable of selecting asecond state in which an electric field for directing a second polaritycharge from the well region to the charge storage region is formed.Here, the first polarity charge means a positive charge typified by ahole or a negative charge typified by an electron, whereas the secondpolarity charge means an electrical charge opposite in polarity to thefirst polarity charge.

According to the above, the first negative voltage is applied to thefirst well region to thereby form the reverse-direction voltage appliedstate (reverse bias state) between the second gate electrode and thememory electrode near the second gate electrode, thereby making itpossible to generate hot holes and hot electrons by band-to-bandtunneling. The electric field for directing the first polarity charge,e.g., hot holes from the well region side to the charge storage regionis formed to thereby produce an avalanche of the hot holes, so that arelatively large number of the hot holes are injected into the chargestorage region.

In the first state, a reverse bias state that is much larger than thatat the occurrence of the hot holes or the like by the band-to-bandtunneling is formed between the second gate electrode and the memoryelectrode near the second gate electrode, to thereby enable thegeneration of a larger number of avalanche hot holes. Thus, the largernumber of avalanche hot holes are injected into the charge storageregion, so that the time required to inject the holes can be shortenedand the time required to write or erase information can be shortened.

Here, a reverse bias voltage between a reverse bias voltage of a pnjunction at the time that the hot holes or the like occur by theband-to-band tunneling, and a reverse bias voltage of a pn junction atthe time that avalanche hot holes greater than those are produced, isreferred to as a “junction withstand voltage (junction withstand)”.Accordingly, the state of a reverse bias, that is much larger than whenthe hot holes or the like occur by the band-to-band tunneling, may begrasped or taken as the state of application of a reverse voltage thatis near to or greater than the junction withstand. If an attempt is madeto define the junction withstand voltage quantitatively, then a reversebias voltage, at the time that a backward or reverse current, of theorder of an allowable leakage current, that is allowed to flow into achannel of a MIS (Metal Insulate Semiconductor) held in an off stateflows through a pn junction (also simply called a “junction”), can bedefined as the junction withstanding. In the present specification, thejunction withstand does not mean a junction breakdown voltage.

Since the well region is set to a negative voltage when the state of thereverse bias near or greater than the junction withstand voltage isformed, the voltage to be applied to the corresponding memory electrodecan be made lower than when the voltage of the well region is set to acircuit's ground voltage. Thus, even when read circuits such as a senseamplifier, etc. are connected to the corresponding memory electrode,there is no need to constitute those read circuits using high withstandvoltage MIS transistors.

The second gate electrode is electrically isolated from the first gateelectrode (so-called split gate structure). Therefore, even if a highvoltage is applied to the second gate electrode to form the first stateor the second state, a withstand voltage of the first gate electrode isnot affected thereby. Thus, there is no need to form the insulating filmof the first gate electrode with a high-withstand film thickness. Forexample, the insulating film of the first gate electrode can be maderelatively thin in a manner similar to a logic MIS transistor. Thus, theGm of a MIS transistor section of a first gate electrode portion in thememory cell transistor can be made relatively large, and the amount of asignal current passing through a channel portion directly below thefirst gate electrode can be made large during the operation of readingmemory information, even if the voltage to be applied to the first gateelectrode is not made high in particular.

In order to prevent breakdown of the insulating film of the first gateelectrode, that is formed relatively thin, in a manner similar to thelogic MIS transistor when a negative voltage is applied to the wellregion upon hot-hole injection where the insulating film of the firstgate electrode is made relatively thin in a manner similar to the logicMIS transistor, a negative voltage lower than the circuit's groundvoltage may preferably be applied within a withstand voltage rangethereof.

When the memory cell transistor is configured as a memory cell forstoring binary information, one first gate electrode is provided nearthe one memory electrode, and one second gate electrode and one chargestorage region are respectively provided near the other memory electrodeto thereby constitute the corresponding memory cell transistor. Thememory cell transistor is capable of storing binary informationaccording to the difference between the amount of the first polaritycharge and the amount of the second polarity charge, each of which isinjected into the corresponding charge storage region. For example,electrons are injected into the charge storage region to form a highthreshold voltage state (e.g., erase state), and hot electrons areinjected into the charge storage region into which the electrons havebeen injected to neutralize the electrons, whereby a low thresholdvoltage state (e.g., write state) is formed.

When the memory cell transistor is configured as a memory cell forstoring quaternary information, the second gate electrode and the chargestorage region are provided near the memory electrode therefor, and onefirst gate electrode is provided in a region between a pair of thesecond gate electrodes. The memory cell transistor is capable of storingquaternary information according to the difference between the amount ofa first polarity charge and the amount of a second polarity charge, bothof which are injected into the pair of the charge storage regions. If adepletion layer much expanded to the drain side is taken intoconsideration in the case of a read operation for the memory celltransistor storing the quaternary information, e.g., where a logic valueof memory information is determined according to the presence or absenceof a current flowing from a drain to a source electrode in an n channeltype memory cell transistor, then a MIS transistor section of a chargestorage region portion placed on the source electrode side will have aconductance corresponding to its threshold voltage state. The MIStransistor section of the charge storage region portion placed on thedrain side does not substantially fulfill its function as a switchregardless of its threshold voltage. Thus, the quaternary determinationof the memory information can be made based on the presence or absenceof a current flowing in a channel region when one memory electrode isused as the drain, and the presence or absence of a current flowing inthe channel region when the other memory electrode is used as the drain.

[2] <<Sprit Gate/Antipolarity Charge Injection/Negative SubstratePotential>>

A semiconductor integrated circuit device according to a specific aspectof the present invention has a memory cell transistor provided on asemiconductor substrate, and an access circuit therefor providedthereon. The memory cell transistor includes a pair of memory electrodesof which one serves as a source electrode and the other serves as adrain electrode, and a channel region interposed between the pair ofmemory electrodes, both of which are provided in a first well region ofthe semiconductor substrate, and includes, on the channel region, afirst gate electrode disposed near the one memory electrode with aninsulating film interposed between the channel region and the first gateelectrode, and a second gate electrode disposed near the other memoryelectrode on the channel region with an insulating film and a chargestorage region interposed between the second gate electrode and thechannel region and electrically isolated from the first gate electrode.The access circuit is capable of selecting a first state in which afirst negative voltage is applied to the first well region to therebyapply a reverse voltage between the memory electrode near the secondgate electrode and the first well region and to apply a voltage forforming an electric field for directing a first polarity charge from thewell region side to the charge storage region to the second gateelectrode. Further, the access circuit is capable of selecting a secondstate in which a voltage for forming an electric field for directing asecond polarity charge to the charge storage region is applied to thesecond gate electrode and first well region.

In the first state, a state (reverse bias state) of application of areverse voltage near or greater than or equal to a junction withstandvoltage, for example, may be formed between the second gate electrodeand the memory electrode near the second gate electrode.

According to the above means, the first negative voltage is applied tothe first well region to thereby form the reverse-direction voltageapplied state (reverse bias state) between the second gate electrode andthe memory electrode near the second gate electrode, thereby making itpossible to generate hot holes and hot electrons by band-to-bandtunneling. The electric field for directing the first polarity charge,e.g., hot holes from the well region side to the charge storage regionis formed to thereby produce an avalanche of the hot holes, so that arelatively large number of the hot holes are injected into the chargestorage region.

In the first state, the state (reverse bias state) of application of thereverse voltage near or greater than or equal to the junction withstandvoltage, for example, is formed between the second gate electrode andthe memory electrode near the second gate electrode to thereby enablethe occurrence of a larger number of avalanche hot holes. Thus, a largernumber of avalanche hot holes are injected into the corresponding chargestorage region, so that the time required to inject the holes can beshortened and the time required to write or erase information can beshortened.

Since the well region is set to a negative voltage when the state of thereverse bias greater than the junction withstand is formed, the voltageto be applied to the corresponding memory electrode can be made lowerthan when the voltage of the well region is set to a circuit's groundvoltage. For example, when the access circuit comprises a first MIStransistor having a relatively thin gate insulating film, and a secondMIS transistor having a relatively thick gate insulating film, theaccess circuit sets a voltage applied to the memory electrode near thesecond gate electrode as a first operation power voltage (Vdd) of acircuit comprising the first MIS transistor in order to form the firststate. Thus, even when read circuits, such as a sense amplifier, etc.are connected to the corresponding memory electrode, there is no need toconstitute those read circuits of high withstand voltage MIStransistors.

Since the second polarity charge, e.g., an electric field for injectingelectrons, is formed between the well region and the second gateelectrode, the field intensity is not biased or extremely small in itsbias at opposite bottom portions in the charge storage region, and theuniform injection of the second polarity charge into the charge storageregion is easy, thus making it possible to prevent the occurrence ofpartial leftovers from erasure or partial leftovers from writing. Thepossibility of there being partial leftovers from erasure or writingwill manifest when a nonconductive trap film or the like is adopted forthe charge storage region.

The second gate electrode is electrically isolated from the first gateelectrode (so-called split gate structure). Therefore, even if a highvoltage is applied to the second gate electrode to form the first stateor the second state, the withstand voltage of the first gate electrodeis not affected thereby. Thus, there is no need to form the insulatingfilm of the first gate electrode with a high-withstand film thickness.For example, the insulating film of the first gate electrode can be maderelatively thin in a manner similar to a logic MIS transistor. Thus, theGm of a MIS transistor section of a first gate electrode portion in thememory cell transistor can be made relatively large, and the amount of asignal current passing through a channel portion directly below thefirst gate electrode can be made large in the case of reading memoryinformation, even if the voltage to be applied to the first gateelectrode is not made high in particular.

In order to prevent breakdown of the insulating film of the first gateelectrode that is formed relatively thin in a manner similar to thelogic MIS transistor when a negative voltage is applied to the wellregion upon hot-hole injection where the insulating film of the firstgate electrode is made relatively thin in a manner similar to the logicMIS transistor, a negative voltage lower than the circuit's groundvoltage, e.g., a second negative voltage smaller than the first negativevoltage in absolute value, may preferably be applied to the first gateelectrode. For example, the setting of the second negative voltage to avoltage (−Vcc) equal to the first operation power voltage in absolutevalue is most suitable. According to it, the first negative voltage maypreferably be set to, for example, a voltage (−nVcc) equal to severaltimes the first operation power voltage in absolute value.

Assuming that the electric field formed in the second state is anelectric field for directing the second polarity charge from the wellregion to the charge storage region, charges opposite to each other inpolarity are injected from the well region so that so-calledwriting/erasing can be performed. In the second state, for example, apositive voltage is applied to the second gate electrode, and acircuit's ground voltage is applied to the first well region. Thus, itis not necessary to consider a trade-off between prevention of undesiredcharge leakage and satisfactory charge pull-out or drawing performanceat the time of memory information rewriting with respect to theinsulating film between the second gate electrode and the charge storageregion. Thus, when the charge storage region is constituted of, forexample, an ONO structure, no problem occurs even if the oxide film(insulating film) on the upper side (near the second gate electrode) isformed thicker than the one on the lower side (on the well region side).The undesired charge leakage developed via the second gate electrode canbe easily reduced.

The circuit's ground voltage may preferably be supplied to the memoryelectrode near the second gate electrode in the second state.

If attention is directed to the operation of reading memory information,then the access circuit may further be capable of selecting a thirdstate in which the second gate electrode is set to the circuit groundvoltage, the first gate electrode is set to the first operation powervoltage, and a current is allowed to flow in the channel region.

The charge storage region can make use of a nonconductive charge trapfilm, an insulating film having conductive particles, or a conductivefloating gate electrode covered with an insulating film, or the like.

When the access circuit comprises a first MIS transistor having arelatively thin gate insulating film, and a second MIS transistor havinga relatively thick gate insulating film, the insulating film for thefirst gate electrode may be thinner than the insulating film for thesecond gate electrode. For example, the insulating film for the firstgate electrode may be made equal to the gate insulating film of thefirst MIS transistor in thickness.

The semiconductor integrated circuit device may further include a logiccircuit connected to the access circuit and comprising the first MIStransistor. The logic circuit may be provided with, for example, a CPUand a RAM.

[3] <<Sprit Gate/Antipolarity Charge Injection/Negative SubstratePotential>>

A semiconductor integrated circuit device according to another specificaspect of the present invention has a memory cell transistor provided ona semiconductor substrate, and an access circuit therefor providedthereon. The memory cell transistor includes, in a first well region ofthe semiconductor substrate, a pair of memory electrodes (10, 11), oneof which serves as a source electrode and the other serves as a drainelectrode, and a channel region interposed between the pair of memoryelectrodes, and includes, on the channel region, a first gate electrode(3) disposed near a region for the one memory electrode with aninsulating film (2) interposed between the channel region and the firstgate electrode, and a second gate electrode (8) disposed near a regionfor the other memory electrode on the channel region with an insulatingfilm (5, 7) and a charge storage region (6) interposed between thesecond gate electrode and the channel region and electrically isolatedfrom the first gate electrode. The access circuit is capable ofselecting a first operation in which a negative voltage for forming areverse bias state between the second gate electrode and the memoryelectrode near the second gate electrode is applied to the first wellregion to thereby inject a first polarity charge into the charge storageregion. Further, the access circuit is capable of selecting a secondoperation in which a positive voltage is applied to the second gateelectrode to thereby inject a second polarity charge into the chargestorage region.

In the first operation, a state of a reverse bias near or greater than ajunction withstand voltage may be formed between the memory electrodenear the second gate electrode and the first well region by the negativevoltage.

When the access circuit comprises a first MIS transistor having arelatively thin gate insulating film and a second MIS transistor havinga relatively thick gate insulating film, the access circuit may set thevoltage applied to the memory electrode near the second gate electrodeas a first operation power voltage for a circuit comprised of the firstMIS transistor.

The access circuit may preferably apply a second negative voltagesmaller in absolute value than the first negative voltage to the firstgate electrode upon the first operation. The second negative voltage maybe a voltage equal to the first operation power voltage in absolutevalue. The first negative voltage may be a voltage equal to severaltimes the first operation power voltage.

The access circuit is capable of injecting hot electrons into thecorresponding charge storage region by application of a second negativevoltage that is larger in absolute value than the first negative voltageto the second gate electrode upon the first operation.

The access circuit applies a circuit's ground voltage to thecorresponding well region and applies the circuit's ground voltage tothe corresponding memory electrode near the second gate electrode uponthe second operation, thereby making it possible to inject electronsinto the charge storage region from the well region.

In the operation of reading memory information, the access circuit mayfurther be capable of selecting a third state in which the second gateelectrode is set to the circuit's ground voltage, the first gateelectrode is set to the first operation power voltage, and a current isallowed to flow in the channel region.

In the first operation, for example, a reverse bias state near orgreater than a junction withstand voltage is formed between the secondgate electrode and the memory electrode near the second gate electrode,so that the access circuit is capable of generating a larger number ofhot holes. Thus, the larger number of hot holes are injected into thecorresponding charge storage region, and hence the time required toinject the holes can be shortened and the time required to write orerase information can be shortened.

[4] <<Sprit Gate/Antipolarity Charge Injection/Negative SubstratePotential>>

A semiconductor integrated circuit device according to a furtherspecific aspect of the present invention has a memory cell transistor, afirst MIS transistor relatively thin in gate insulating film, and asecond MIS transistor relatively thick in gate insulating film, all ofwhich are provided on a semiconductor substrate. The memory celltransistor includes, in a first well region of the semiconductorsubstrate, a pair of memory electrodes (10, 11), one of which serves asa source electrode and the other serves as a drain electrode, and achannel region interposed between the pair of memory electrodes, andincludes, on the channel region, a first gate electrode (3) disposednear a region for the one memory electrode with an insulating film (2)interposed between the channel region and the first gate electrode, anda second gate electrode (8) disposed near a region for the other memoryelectrode on the channel region with an insulating film (5, 7) and acharge storage region (6) interposed between the second gate electrodeand the channel region and electrically isolated from the first gateelectrode, and is capable of storing information different according tothe difference between amounts of a first polarity charge and a secondpolarity charge each injected into the charge storage region. Theinsulating film placed below the first gate electrode is equal inthickness to the gate insulating film of the first MIS transistor. Thewell region is supplied with a negative voltage for forming, forexample, a state of a reverse bias near or greater than a junctionwithstand voltage between the second gate electrode and the memoryelectrode near the second gate electrode when the first polarity chargeis injected into the corresponding charge storage region. The secondgate electrode is supplied with a positive voltage when the secondpolarity charge is injected into the charge storage region.

[5] <<Multi-Valued Memory Cell>>

A semiconductor integrated circuit device according to a still furtheraspect of the present invention has a memory cell transistor provided ona semiconductor substrate, and an access circuit therefor providedthereon. The memory cell transistor includes, in a first well region ofthe semiconductor substrate, a pair of memory electrodes (128), one ofwhich serves as a source electrode and the other serves as a drainelectrode, and a channel region interposed between the pair of memoryelectrodes, and includes, on the channel region, memory gate electrodes(127) separately disposed near the respective memory electrodes throughinsulating films (124, 126) and charge storage regions (125), and acontrol gate electrode disposed between both the memory gate electrodeswith an insulating film (122) interposed therebetween and electricallyisolated from the memory gate electrodes. The access circuit is capableof selecting a first state in which a negative voltage is applied to thefirst well region to form a state of a reverse bias close to or greaterthan, for example, a junction withstand voltage between the well regionand the one memory electrode and to form an electric field for directinga first polarity charge from the well region side to the charge storageregion on the one memory electrode side, a second state in which anelectric field for directing a second polarity charge from the wellregion to the charge storage regions of both the memory gate electrodesis formed, and a third state in which a current is allowed to mutuallyflow from the one memory electrode to the other memory electrode throughthe channel region.

<<Another Viewpoint of Multi-Valued Memory Cell>>

A semiconductor integrated circuit device according to a still furtherspecific aspect of the present invention has a memory cell transistorprovided on a semiconductor substrate, and an access circuit thereforprovided thereon. The memory cell transistor includes, in a first wellregion of the semiconductor substrate, a pair of memory electrodes(128), one of which serves as a source electrode and the other serves asa drain electrode, and a channel region interposed between the pair ofmemory electrodes, and includes, on the channel region, memory gateelectrodes (127) separately disposed near the respective memoryelectrodes through insulating films (124, 126) and charge storageregions (125), and a control gate electrode (123) disposed between boththe memory gate electrodes with an insulating film (122) interposedtherebetween and electrically isolated from the memory gate electrodes.The access circuit is capable of selecting a first operation in which anegative voltage is applied to the first well region to thereby form astate of a reverse bias close to or greater than, for example, ajunction withstand voltage between the first well region and the onememory electrode and inject a first polarity charge into the one chargestorage region, a second operation in which a positive voltage isapplied to both the memory gate electrodes to thereby inject a secondpolarity charge from the well region to both the charge storage regions,and a third operation in which a current is allowed to mutually flowfrom the one memory electrode to the other memory electrode through thechannel region.

[6] <<<<Sprit Gate/Antipolarity Charge Injection/Negative SubstratePotential>>

A semiconductor integrated circuit device according to a still furtherspecific aspect of the present invention has a memory cell transistor, afirst MIS transistor having a relatively thin gate insulating film, anda second MIS transistor having a relatively thick gate insulating film,all of which are provided on a semiconductor substrate. The memory celltransistor includes a source region, a drain region and a channel regioninterposed between the source region and the drain region, all of whichare provided within a first well region of the semiconductor substrate,and includes a first gate electrode (CG) disposed on one sides of thesource region and drain region, a second gate electrode disposed on theother sides of the source region and drain region, a first gateinsulating film (46, 129) formed between the channel region and thefirst gate electrode, a charge storage region (6, 125) formed betweenthe channel region and the second gate electrode, and an insulating filmfor electrically isolating the first gate electrode and the second gateelectrode, all of which are provided over the channel region. In thecase of a write or erase operation of the memory cell transistor, anegative voltage having a value smaller in absolute value than one equalto several times a power voltage Vcc of a circuit comprising the firstMIS transistor, and a ground voltage for the circuit are applied to thefirst well region to thereby inject carriers into the correspondingcharge storage region.

<<Application of Negative Voltage (−Vcc) to CG>>

In the case of a write or erase operation of the memory cell transistor,a negative first voltage is applied to the second gate electrode and anegative second voltage, that is smaller than the negative first voltagein absolute value, is applied to the first gate electrode to therebyinject holes into the corresponding charge storage region.

[7] <<Negative Voltage to MG>Negative Voltage to CG, Hole Injection>>

A semiconductor integrated circuit device according to a still furtherspecific aspect of the present invention has a memory cell transistor.The memory cell transistor includes a source region, a drain region, anda channel region interposed between the source region and the drainregion, all of which are provided within a first well region of asemiconductor substrate, and includes, on the channel region, a firstgate electrode (CG), a second gate electrode (MG), a first gateinsulating film (46, 129) formed between the channel region and thefirst gate electrode, a charge storage region (6, 126) formed betweenthe channel region and the second gate electrode, and an insulating filmfor electrically isolating the first gate electrode and the second gateelectrode. In the case of a write or erase operation of the memory celltransistor, a negative first voltage is applied to the second gateelectrode and a negative second voltage, that is smaller than thenegative first voltage in absolute value, is applied to the first gateelectrode to thereby inject holes into the corresponding charge storageregion.

If the second voltage applied to the CG is set to a low voltage like−Vcc, then a control system for the first gate electrode can be formedof a low withstand voltage MIS circuit. For example, the first gateelectrode is electrically connected to a first driver circuit fordriving a gate control line, through the gate control line. The firstdriver circuit comprises a low withstand voltage transistor (powervoltage MIS transistor). The first gate insulating film is formed by agate insulating film forming process for the low withstand voltagetransistor.

The charge storage region comprises a nonconductive charge trap film.The charge storage region is formed over the channel region with a firstinsulating film being interposed therebetween. The first gate electrodeconstitutes a control gate electrode. The second gate electrodeconstitutes a memory gate electrode.

[8] <<Vcc to Source or Drain, Negative Voltage to Well, Hole Injection>>

A semiconductor integrated circuit device according to a still furtherspecific aspect of the present invention has a memory cell transistor, afirst MIS transistor having a relatively thin gate insulating film, anda second MIS transistor having a relatively thick gate insulating film,all of which are provided on a semiconductor substrate. The memory celltransistor includes a source region, a drain region, a channel regioninterposed between the source region and the drain region, a gateelectrode, and a charge storage region (6, 125) formed between thechannel region and the gate electrode, all of which are provided withina first well region of the semiconductor substrate. In the case of awrite or erase operation of the memory cell transistor, a negative firstvoltage is applied to the gate electrode, a negative second voltage notgreater than the first voltage in absolute value is applied to the firstwell region, and a third voltage (Vcc) not greater in absolute valuethan a power voltage (Vcc) of a circuit made of the first MIS transistoris applied to the source or drain region to thereby inject holes intothe corresponding charge storage region.

<<Hole Generation at Applied Voltage≥Junction Withstand Voltage>>

In the case of a write or erase operation of the memory cell transistor,the difference in potential between the third voltage (Vcc) and thesecond voltage (−2Vcc) is close to a junction withstand voltage of thesource or drain region and is capable of generating holes byband-to-band tunneling.

If the third voltage applied to the drain is set to a low voltage likeVcc, then a bit-line circuit connected to the drain can be formed of alow withstand voltage MIS circuit. For example, the source region or thedrain region is electrically connected to a first driver circuit fordriving a bit control line, through the bit control line. The firstdriver circuit comprises a low withstand voltage transistor (powervoltage MIS transistor). The charge storage region is formed of anonconductive charge trap film over the channel region, with a firstinsulating film being interposed therebetween.

[9] <<Overlap Structure of CG and MG for Gate of Peripheral MOSTransistor, FIGS. 24 and 55>>

A semiconductor integrated circuit device according to a still furtherspecific aspect of the present invention has a memory cell transistor,and at least one peripheral circuit transistor. The memory celltransistor includes, in a memory cell forming region of a semiconductorsubstrate, a source region, a drain region, a channel region interposedbetween the source region and the drain region, a first gate electrodeand a second gate electrode disposed on the channel region, a first gateinsulating film (46, 129) formed between the channel region and thefirst gate electrode, a charge storage region (6, 125) formed betweenthe channel region and the second gate electrode, and an insulating filmfor electrically isolating the first gate electrode and the second gateelectrode. The peripheral circuit transistor (power voltage MIStransistor, high voltage MIS transistor) has a gate electrode on aperipheral circuit transistor forming region of the semiconductorsubstrate. The gate electrode of the peripheral circuit transistorcomprises a film formed by laminating a first conductive film lying inthe same layer as the first gate electrode, and a second conductive filmlying in the same layer as the second gate electrode.

The charge storage region comprises a nonconductive charge trap film,for example. The first gate electrode constitutes the control gateelectrode. The second gate electrode constitutes a memory gate electrodeand is formed on side walls of the control gate electrode through aninsulating film in the form of sidewall spacer shapes (8, 62, 98, 127).The second conductive film is formed on the first conductive film.

The above at least one peripheral transistor includes a low withstandvoltage transistor (power voltage MIS transistor) operated at a powervoltage (Vcc), and a high withstand voltage transistor (high voltage MIStransistor) operated at a voltage higher than the power voltage.

The first gate insulating film (46, 129) is formed by a gate insulatingfilm forming process for the low withstand voltage transistor.

[10] <<Manufacturing Process of Section [9]>>

A method of manufacture of a semiconductor integrated circuit device,according to the present invention, includes the steps of forming afirst conductive film over a memory cell forming region and a peripheralcircuit transistor forming region of a semiconductor substrate,patterning the first conductive film lying over the memory cell formingregion to form a first conductive pattern which serves as a first gateelectrode of a memory cell and leaving the first conductive film overthe peripheral circuit transistor forming region, forming a secondconductive film on the memory cell forming region and the firstconductive film in the peripheral circuit transistor forming region, andetching the second conductive film to form each second gate electrode ofthe memory cell on at least side walls of the first conductive pattern,and forming a gate electrode of each peripheral circuit transistorcomprising the second conductive film and first conductive film over theperipheral circuit transistor forming region.

The memory cell includes, in a memory cell forming region of asemiconductor substrate, a source region, a drain region, a channelregion interposed between the source region and the drain region, acontrol gate electrode and a memory gate electrode disposed on thechannel region, a first gate insulating film (46, 129) formed betweenthe channel region and the control gate electrode, and a charge storageregion (6, 125) formed between the channel region and the memory gateelectrode. The first gate electrode constitutes the control gateelectrode. The second gate electrode constitutes the memory gateelectrode.

The peripheral circuit transistors include a low withstand voltagetransistor (power voltage MIS transistor) operated at a power voltage,and a high withstand voltage transistor (high voltage MIS transistor)operated at a voltage higher than the power voltage. The first gateinsulating film is formed by a gate insulating film forming step for thelow withstand voltage transistor.

The second gate electrode is formed on side walls of the first gateelectrode through an insulating film in the form of each sidewall spacershape (8, 62, 98, 127).

An electrode withdrawal portion (200) of the second gate electrode isformed in the forming step of the second gate electrode.

The method further includes a step of patterning the first conductivepattern after the formation of the second gate electrode to thereby formthe first gate electrode.

[11] <<Isolation of Silicide Layers 14 by Spacers 12 and 13>>

A semiconductor integrated circuit device according to a still furtherspecific aspect of the present invention has at least one memory cell.The memory cell includes a source region, a drain region, and a channelregion interposed between the source region and the drain region, all ofwhich are provided within a semiconductor region, and includes a firstgate electrode (CG), a second gate electrode (MG), and an insulatingfilm for electrically isolating the first gate electrode and the secondgate electrode, all of which are provided on the channel region. Thechannel region comprises a first channel region and a second channelregion. A first gate insulating film is provided between the firstchannel region and the first gate electrode. A second gate insulatingfilm is provided between the second channel region and the second gateelectrode. The second gate electrode is formed higher than the firstgate electrode. A silicide layer (14) for the second gate electrode anda silicide layer (14) for the first gate electrode are electricallyisolated by sidewall spacers (13) each comprised of an insulating film,in self-alignment with side walls of the second gate electrode.Undesired short-circuits in both silicide layers are easily and reliablyprevented.

[12] <<Height: CG<MG, MG: Low Resistance>>

A semiconductor integrated circuit device according to a still furtherspecific aspect of the present invention has at least one memory cell.The memory cell includes a source region, a drain region, and a channelregion interposed between the source region and the drain region, all ofwhich are provided within a semiconductor region, and includes a firstgate electrode (CG), a second gate electrode (MG), and an insulatingfilm for electrically isolating the first gate electrode and the secondgate electrode, all of which are provided on the channel region. Thechannel region comprises a first channel region and a second channelregion. A first gate insulating film is provided between the firstchannel region and the first gate electrode. A second gate insulatingfilm is provided between the second channel region and the second gateelectrode. The second gate electrode is formed on side walls of thefirst gate electrode through an insulating film in the form of eachsidewall spacer shape (8, 62, 98, 127). The thickness of the second gateelectrode is thicker than the thickness of the first gate electrode, andthe height on a substrate surface, of the second gate electrode ishigher than the height on the substrate surface, of the first gateelectrode. When the first and second gate electrodes are silicidized,undesired short-circuits in both silicide layers are easily and reliablyprevented.

In order to reduce the resistance value of the second gate electrode, asilicide layer (14) may preferably be formed in the second gateelectrode. More specifically, sidewall spacers (12, 13), each comprisedof an insulating film, are formed in self-alignment with side walls onboth sides of the second gate electrode. The silicide layer (14) for thesecond gate electrode and the silicide layer (14) for the first gateelectrode are electrically isolated by the sidewall spacer (13) disposedon one side of both sides. The silicide layer (14) for the second gateelectrode and a silicide layer (14) for the source region or the drainregion are electrically isolated by the sidewall spacer (12) on theother side thereof. The silicide layer (14) for the first gate electrodeand the silicide layer (14) for the source region or the drain regionare electrically isolated by sidewall spacers (12) each made of aninsulating film, which are formed on the side walls of the first gateelectrode on a self-alignment basis.

The second gate insulating film includes a nonconductive charge trapfilm corresponding to, for example, a charge storage region (6, 125).The first gate electrode (CG) constitutes a control gate electrode ofthe memory cell. The second gate electrode (MG) constitutes a memorygate electrode of the memory cell and is formed on side walls of thecontrol gate electrode through an insulating film in the form of eachsidewall spacer shape (8, 62, 98, 127).

[13] <<Manufacturing Process of Section [11]>>

A method of manufacture of a semiconductor integrated circuit deviceincludes the steps of forming a first conductive film (51) over a memorycell forming region of a semiconductor substrate and forming aninsulating film (50) on the first conductive film (see FIG. 19), etchingthe insulating film and the first conductive film to form a firstconductive pattern which serves as a first gate electrode (CG) of amemory cell (see FIG. 20), forming a second gate electrode (62) of thememory cell on side walls of the first conductive pattern, removing theinsulating film (50) on the first conductive pattern (see FIG. 24),forming sidewall spacers (69), each made of an insulating film on sidewalls of the second gate electrode (62) on a self-alignment basis (seeFIG. 26), and forming a silicide layer (77) for each of the firstconductive pattern and the second gate electrode (62) in self-alignmentwith respect to the sidewall spacers (69) (see FIG. 27).

More specifically, in the sidewall spacer (69) forming step (see FIG.26), the sidewall spacers (69) are formed on the side walls on bothsides of the second gate electrode and side walls of the first gateelectrode. The silicide layer (77) for the second gate electrode and thesilicide layer (77) for the first gate electrode are electricallyisolated by the sidewall spacer (69) disposed on one side of both sides.The silicide layer (77) for the second gate electrode and a silicidelayer (77) for a source region or a drain region are electricallyisolated by the sidewall spacer (69) on the other side thereof. Thesilicide layer (77) for the first gate electrode and the silicide layer(77) for the source region or the drain region are electrically isolatedby sidewall spacers (69) formed on the side walls of the first gateelectrode.

More specifically, a gate electrode of each peripheral circuittransistor is formed of a film obtained by laminating a conductive filmlying in the same layer as the first conductive film, and a secondconductive film lying in the same layer as the memory gate electrode.

The silicide layer forming step can serve as a silicide layer formingstep for each peripheral MIS transistor. Namely, sidewall spacers areformed on side walls of the gate electrode of each peripheral circuittransistor in the sidewall spacer (69) forming step. A silicide layer isformed on the gate electrode of the peripheral circuit transistor in thesilicide layer (77) forming step.

More specifically, the memory cell includes, in a memory cell formingregion of a semiconductor substrate, a source region, a drain region, achannel region interposed between the source region and the drainregion, a control gate electrode disposed near one of the source anddrain regions, a memory gate electrode disposed near the other of thesource and drain regions, a first gate insulating film (46, 129) formedbetween the channel region and the control gate electrode, and a chargestorage region (6, 125) formed between the channel region and the memorygate electrode. The first gate electrode constitutes the control gateelectrode. The second gate electrode constitutes the memory gateelectrode.

[14] <<Memory Cell Structure Wherein Each Memory Gate Electrode isFormed in Self-Alignment with Respect to Each Spacer (100) (See FIGS. 35Through 39>>

A semiconductor integrated circuit device has at least one memory cell.The memory cell includes a source region, a drain region, and a channelregion interposed between the source region and the drain region, all ofwhich are provided within a semiconductor region, and includes a firstgate electrode (101), a second gate electrode (98), and an insulatingfilm for electrically isolating the first gate electrode and the secondgate electrode, all of which are provided on the channel regioninterposed between the source and drain regions. The channel regioncomprises a first channel region and a second channel region. A firstgate insulating film (92) is provided between the first channel regionand the first gate electrode. A second gate insulating film (95, 96, 97)is provided between the second channel region and the second gateelectrode. The second gate electrode (98) is formed higher than thefirst gate electrode (101). The first gate electrode (101) is formed inself-alignment with respect to sidewall spacers (100) each made of aninsulating film, which are formed in self-alignment with respect to sidewalls of the second gate electrode (98).

More specifically, sidewall spacers (100), each comprising an insulatingfilm, are formed in self-alignment with the side walls on both sides ofthe second gate (see FIG. 36). The second gate insulating film (95, 96,97) is formed in self-alignment with respect to the sidewall spacer(100) on one side of both sides (see FIG. 38). The first gate electrode(101) is formed in self-alignment with respect to the sidewall spacer(100) on the other side thereof.

More specifically, the second gate insulating film includes anonconductive charge trap film corresponding to a charge storage region(96). The first gate electrode (101) constitutes a control gateelectrode. The second gate electrode (98) constitutes a memory gateelectrode and is formed on each side wall of the control gate electrodethrough an insulating film in sidewall spacer fashion (98).

[15] <<Manufacturing Method of Section [14]>>

A method of manufacture of a semiconductor integrated circuit deviceincludes the steps of forming a first conductive film (93) over a memorycell forming region of a semiconductor substrate and forming aninsulating film (94) on the first conductive film (see FIGS. 19 and 35),etching the insulating film and the first conductive film to form afirst conductive pattern which serves as a first gate electrode of amemory cell (see FIGS. 20 and 35), forming a second gate electrode (98)of the memory cell on side walls of the first conductive pattern (seeFIG. 35), removing the insulating film on the first conductive pattern(see FIG. 36), forming sidewall spacers (100), each comprised of aninsulating film, in self-alignment with side walls of the second gateelectrode (98) (see FIG. 36), and etching the first conductive patternin self-alignment with respect to the sidewall spacers (100) to formeach first gate electrode (101) (see FIG. 38).

More specifically, a second gate insulating film (96) is formed betweenthe second gate electrode (98) and the semiconductor substrate. Thesidewall spacers (100) are formed on the side walls on both sides of thesecond gate electrode (see FIG. 36). The second gate insulating film isformed in self-alignment with respect to the sidewall spacer on one sideof both sides (see FIG. 38). The first gate electrode (101) is formed inself-alignment with respect to the sidewall spacer on the other sidethereof.

More specifically, a gate electrode of each peripheral circuittransistor is formed of a film obtained by laminating a conductive filmlying in the same layer as the first conductive film, and a secondconductive film lying in the same layer as the memory gate electrode.

More specifically, the second gate insulating film includes anonconductive charge trap film corresponding to a charge storage region(96). The first gate electrode (101) constitutes a control gateelectrode. The second gate electrode (98) constitutes a memory gateelectrode and is formed on side walls of the control gate electrodethrough an insulating film in sidewall spacer fashion (98).

[16] <<Threshold Voltage Control>>

A semiconductor integrated circuit device according to a still furtheraspect of the present invention has a basic structure similar to onesdescribed up to now, i.e., a memory cell transistor provided on asemiconductor substrate, and an access circuit therefor providedthereon. The memory cell transistor includes a pair of memoryelectrodes, one of which serves as a source electrode and the otherserves as a drain electrode, and a channel region interposed between thepair of memory electrodes, both of which are provided in a first wellregion of the semiconductor substrate, and includes, on the channelregion, a first gate electrode disposed near the corresponding memoryelectrode with a first gate insulating film interposed between the firstgate electrode and the channel region, and a second gate electrodeformed on the channel region with a second insulating film and a chargestorage region and electrically isolated from the first gate electrode.Further, the first gate electrode is different in conductivity type fromthe second gate electrode. An initial threshold voltage as viewed fromthe first gate electrode and an initial threshold voltage as viewed fromthe second gate electrode are determined so as to be desirable in termsof a read operation. For example, an initial threshold voltage as viewedfrom the second gate electrode is set low in the case of a readoperation, and the voltage to be applied to the second gate electrode isset to a low voltage like a circuit's ground voltage upon reading,thereby making it possible to avoid a reduction in data retentionperformance due to a so-called word line disturbance.

As a still further specific aspect, the thickness of the first gateinsulating film may be formed so that it is thinner than that of thesecond gate insulating film. Further, the first gate electrode may beset to a p type, and the second gate electrode may be set to an n type.At this time, a channel region results in an n type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view illustrating a nonvolatilememory cell transistor applied to a semiconductor integrated circuitdevice according to the present invention;

FIG. 2 is a vertical cross-sectional view of a memory cell according tothe present invention, which is manufactured in the process of mixingwith a logic transistor;

FIG. 3 is a plan view of the memory cell shown in FIG. 2;

FIG. 4 is a plan view illustrating the layout of processing maskpatterns for forming memory gates only in side face portions on thedrain side, of control gates employed in the memory cell according tothe present invention;

FIG. 5 is a cross-sectional view illustrating a voltage-applied state atan erase operation of the memory cell;

FIG. 6 is a cross-sectional view illustrating a voltage-applied state ata write operation of the memory cell;

FIG. 7 is a cross-sectional view illustrating a state of a readoperation of the memory cell;

FIG. 8 is a block diagram illustrating a data processor with an on-chipflash memory;

FIG. 9 is a block diagram illustrating the details of the flash memory;

FIG. 10 is a circuit diagram illustrating the state of a memory array atan erase operation effected on the flash memory;

FIG. 11 is a circuit diagram illustrating the state of the memory arrayat a write operation effected on the flash memory;

FIG. 12 is a circuit diagram illustrating the state of the memory arrayat a read operation effected on the flash memory;

FIG. 13 is a diagram illustrating another bit line structure in a memorycell block;

FIG. 14 is a vertical cross-sectional view showing another example of amemory cell transistor;

FIG. 15 is a fragmentary vertical cross-sectional view of an LSI in astep of the manufacturing process at the time that the logic LSI ismixed with a nonvolatile memory cell as illustrated in FIG. 2 by a 0.13μm process technology;

FIG. 16 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 15;

FIG. 17 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 16;

FIG. 18 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 17;

FIG. 19 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 18;

FIG. 20 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 19;

FIG. 21 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 20;

FIG. 22 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 21;

FIG. 23 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 22;

FIG. 24 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 23;

FIG. 25 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 24;

FIG. 26 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 25;

FIG. 27 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 26;

FIG. 28 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 27;

FIG. 29 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 28;

FIG. 30 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 29;

FIG. 31 is a plan view showing a flat pattern of a memory cell unitcorresponding to FIG. 20;

FIG. 32 is a plan view showing a flat pattern of the memory cell unitcorresponding to FIG. 23;

FIG. 33 is a plan view showing a flat pattern of the memory cell unitcorresponding to FIG. 25;

FIG. 34 is a fragmentary cross-sectional view of an LSI in which changesin another manufacturing method used as an alternative to themanufacturing method illustrated in FIGS. 15 through 29, which adopts amemory cell whose electrode structure is partly changed, are typicallyillustrated;

FIG. 35 is a fragmentary cross-sectional view of an LSI in a step of themanufacturing process for processing both a control gate and a memorygate on a self-alignment basis without depending on their processing bylithography;

FIG. 36 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 35;

FIG. 37 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 36;

FIG. 38 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 37;

FIG. 39 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 38;

FIG. 40 is a vertical cross-sectional view showing a structure in whicha tungsten polycide film is applied to the control gate of the memorycell, as a point different from FIG. 35;

FIG. 41 is a vertical cross-sectional view illustrating a processsection corresponding to FIG. 36 where the structure of FIG. 40 isadopted;

FIG. 42 is a vertical cross-sectional view illustrating a processsection corresponding to FIG. 37 where the structure of FIG. 40 isadopted;

FIG. 43 is a vertical cross-sectional view illustrating, in section, aprocess to be added after the process of FIG. 37 where a cobalt silicidefilm is formed on the side spacer-like memory gate shown in FIG. 35immediately after the process of forming the memory gate;

FIG. 44 is a vertical cross-sectional view illustrating a structure inwhich SiO₂ sidewalls are formed after the step of FIG. 42 to bring thetops of diffusion layers into CoSi salicidation in the case of FIG. 42;

FIG. 45 is a plan layout diagram of a multi-valued memory cell;

FIG. 46 is a plan layout diagram illustrating portions for withdrawal ofcontacts to a control gate and a memory gate of the multi-valued memorycell of FIG. 45;

FIG. 47 is a vertical cross-sectional view illustrating the multi-valuedmemory cell shown in FIG. 45;

FIG. 48 is a circuit diagram illustrating, in an erase-operated state, amemory array in which the multi-valued memory cells each shown in FIG.45 are disposed in matrix form;

FIG. 49 is a circuit diagram illustrating, in a write-operated state,the memory array in which the multi-valued memory cells each shown inFIG. 45 are disposed in matrix form;

FIG. 50 is a circuit diagram illustrating, in a state of a readoperation for a plus direction, the memory array in which themulti-valued memory cells each shown in FIG. 45 are disposed in matrixform;

FIG. 51 is a circuit diagram illustrating, in a state of a readoperation for a reverse direction, the memory array in which themulti-valued memory cells each shown in FIG. 45 are disposed in matrixform;

FIG. 52 is a fragmentary vertical cross-sectional view of an LSI in astep of the manufacturing process of a multi-valued memory cell;

FIG. 53 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 52;

FIG. 54 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 53;

FIG. 55 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 54;

FIG. 56 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 55;

FIG. 57 is a fragmentary vertical cross-sectional view of the LSI in astep of the manufacturing process following FIG. 56;

FIG. 58 is a cross-sectional view illustrating a write operation of anonvolatile memory cell according to a first prior art example;

FIG. 59 is a cross-sectional view illustrating an erase operation of thenonvolatile memory cell according to the first prior art example;

FIG. 60 is a cross-sectional view illustrating a write operation of anonvolatile memory cell according to a second prior art example;

FIG. 61 is a cross-sectional view illustrating an erase operation of thenonvolatile memory cell according to the second prior art example;

FIG. 62 is a cross-sectional view illustrating a read operation of thenonvolatile memory cell according to the second prior art example;

FIG. 63 is a cross-sectional view illustrating a write operation of anonvolatile memory cell according to a third prior art example;

FIG. 64 is a cross-sectional view illustrating an erase operation of anonvolatile memory cell according to a fourth prior art example; and

FIG. 65 is a cross-sectional view illustrating a write operation of thenonvolatile memory cell according to the fourth prior art example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described indetail with reference to the accompanying drawings. Incidentally,components having the same function in all of the drawings arerespectively identified by the same reference numerals, and theirrepetitive description will be omitted. In the following description, aMOS (Metal Oxide Semiconductor) transistor (also simply described as“MOS”) will be used as one example of MIS transistors (or MISFET), whichare generic names for insulated gate field effect transistors.

<<Memory Cell Transistor>>

A nonvolatile memory cell transistor (also simply called a “memorycell”) applied to a semiconductor integrated circuit device according tothe present invention is illustrated in FIG. 1 in the form of a verticalcross-section. A first structural viewpoint related to the memory celltransistor resides in writing and erasure operations based on electron-and hot-hole injection, and a split gate structure. Namely, the memorycell transistor shown in the drawing comprises a read transistor unit orsection (selection transistor section) in which a control gate (controlgate electrode or first gate electrode) 3 is formed in a surface regionof a semiconductor substrate (or well region) 1 with a gate insulatingfilm 2, that is made of, for example, a silicon oxide film, interposedtherebetween, and a memory transistor unit or section in which, forexample, a lower silicon oxide film 5 corresponding to a gate insulatingfilm, a charge storage region 6, and an upper silicon oxide film 7corresponding to an insulating film are laminated on the surface regionof the semiconductor substrate 1 on at least the drain side of thecontrol gate 3, and a memory gate (memory gate electrode or second gateelectrode) 8 is formed thereabove. The charge storage region 6 is aregion for holding information therein and is capable of discontinuouslyperforming, for example, charge retention on a discrete basis. Theholding region is made of, for example, a non-conductive charge trapfilm. As the non-conductive charge trap film, one may used, for example,a silicon nitride film. Since the silicon nitride film is discontinuousand discrete in charge's trap, all of the stored charges do notdisappear and retention characteristics can be enhanced even wherecharge leakage paths, such as pin holes or the like, occur in part ofthe lower silicon film 5 corresponding to the gate insulating film. Thethickness of the upper silicon oxide film 7 is formed so as to bethicker than that of the lower silicon oxide film 5, and the thicknessof the gate insulating film 2 is formed so as to be thinner than thethicknesses of the laminated films 5, 6 and 7. A drain (memory electrodecorresponding to a drain electrode (region)) 10 is formed in the surfaceregion of the semiconductor substrate 1, which is overlapped with thememory gate 8. A source (memory electrode corresponding to a sourceelectrode (region)) 11 is formed in the surface region of thesemiconductor substrate 1, which has overlapped with the control gate 3.While the source and drain of each MOS transistor are generally relativeconcepts based on applied voltages, a memory electrode connected to theupstream side of a current path during a read operation is called a“drain” for convenience herein. It is needless to say that, thoughdescribed later, a circuit may be configured with reference numerals 10and 11 as the source and drain, respectively. The insulating films 5, 6and 7, for electrically separating the control gate 3 and the memorygate 8, are formed therebetween.

Thus, the memory cell transistor includes the control gate 3 formedthrough the gate insulating film 2, the memory gate 8 formed through thegate insulating film 5 and the charge storage region 6, and theinsulating films 5, 6 and 7 for electrically separating the control gate3 and the memory gate 8 from each other, all of which are provided overa channel region (semiconductor substrate or well region) interposedbetween the source 11 and the drain 10.

The memory cell shown in FIG. 1 has a high threshold voltage state(e.g., erase state) obtained by, for example, applying a positivevoltage to only the memory gate 8, injecting electrons 20 from thesemiconductor substrate 1 side by virtue of a tunneling current andtrapping them into the silicon nitride film 6, and it has a lowthreshold voltage state (write state) obtained by applying a positivevoltage to the drain 10 and a negative voltage to at least the memorygate 8 and injecting hot holes developed in the neighborhood of ajunction surface of the drain 10 into the silicon nitride film 6 tothereby neutralize trapped electrons. Incidentally, when one of anegative charge typified by an electron or a positive charge typified bya hole is assumed to be a first polarity charge, a charge opposite inpolarity to the first polarity charge is called a “second polaritycharge”.

A second viewpoint related to the memory cell transistor resides in thefact that a large read current is allowed, in other words, the memorycell transistor can be structurally made common to a logic transistor(power voltage MOS transistor). A vertical cross-section of the memorycell transistor according to the present invention, where it ismanufactured in the process of mixing with the logic transistor (powervoltage MOS transistor), is illustrated in FIG. 2. A plan view thereofis illustrated in FIG. 3. Incidentally, FIG. 2 is a cross-sectional viewtaken along line A-A′ of FIG. 3. The left side of FIG. 2 corresponds toA, and the right side of FIG. 2 corresponds to A′, respectively. Onlythe memory cell transistor is shown in FIGS. 2 and 3, and the mixingprocess will be described later. Each MOS transistor operated at asource or power voltage Vdd will be abbreviated as a power voltage MOStransistor.

In FIG. 2, a control gate (control gate electrode or first gateelectrode) 3 formed, in the same manufacturing process as a gateelectrode of the logic transistor (first MOS transistor having arelatively thin insulating film), over a gate insulating film 2 formedin the same manufacturing process as a gate insulating film of the logictransistor operated at the power voltage, and a memory gate (memory gateelectrode or second gate electrode) 8 provided above a laminated film ofa lower oxide film 5 corresponding to a gate insulating film, a siliconnitride film 6 corresponding to a charge storage region and an upperoxide film 7 corresponding to an insulating film are formed over asurface region of a semiconductor substrate 1 that is made of, forexample, silicon. Incidentally, the thickness of the upper silicon oxidefilm 7 is formed so as to be thicker than that of the lower siliconoxide film 5. A drain (memory electrode corresponding to a drainelectrode) 10 and a source (memory electrode corresponding to a sourceelectrode) 11 are disposed in the surface region of the semiconductorsubstrate 1 so as to overlap with the memory gate 8 and the control gate3, respectively. The control gate 3 and the memory gate 8 arerespectively made of, for example, a silicon film. Since the lower oxidefilm 5 is formed by a thermal oxidation process, for example, a siliconoxide film 4 corresponding to a sidewall insulating film is grown onside face portions of the control gate 3. Thus, the thickness of thesilicon oxide film 4 is formed so as to be thicker than that of thelower oxide film 5, and the withstand voltage between the control gate 3and the memory gate 8 can be enhanced. In FIG. 2, for example, metalsilicide films 14 each made of cobalt silicide (CoSi) or nickel silicide(NiSi) are formed over the control gate 8, the memory gate 8, and thesurface regions of the drain 10 and source 11. They are electricallyisolated (separated) by side spacers 12 and 13 made of an insulatingfilm. Incidentally, since the side spacers 12 and 13 are formed withoutusing a photolithography technology (to be described later) and in thesame process step in a manufacturing process, the manufacturing processsteps can be reduced. An interlayer insulating film 15 is formed so asto cover the memory cell transistor and the logic transistor, and thesurface of the interlayer insulating film 15 has been planarized.Connecting holes 197 and 198 for opening the drain 10 and source 11 aredefined in the interlayer insulating film 15, and metal plugs 16 areembedded into the connecting holes. An interlayer insulating film 17,whose surface has been planarized, is formed on the interlayerinsulating film 15, and a bit line 19 is formed on the interlayerinsulating film 17. The connecting hole 197 for opening the metal plug16 placed over the drain 10 is defined in the interlayer insulating film17, and a metal plug 18 is embedded into the connecting hole 197.Incidentally, the connecting holes 197 and 198 will be described laterwith reference to FIG. 4. Thus, the metal plugs 16 are electricallyconnected to the drain 10 and source 11, respectively. Further, themetal plug 16 formed over the drain 10 is electrically connected to thebit line 19 through the metal plug 18.

In the plan view of the memory cell shown in FIG. 3, a control gate 23(corresponding to the control gate 3), an oxide film 24 (correspondingto the oxide film 5), a silicon nitride film 25 (corresponding to thesilicon nitride film 6), an upper oxide film (corresponding to the upperoxide film 7), a memory gate 27 (corresponding to the memory gate 8),and insulating film side spacers 28 (corresponding to the side spacers12) are disposed in active regions 22 surrounded by device isolationregions so as to extend in a direction (second direction: longitudinaldirection as viewed in the drawing) orthogonal to a direction (firstdirection: transverse direction as viewed in the drawing) in which theactive regions 22 extend. Further, metal plugs 29 (corresponding to themetal plugs 16), are placed over the drain 10 and source 11, and bitlines 30 (each corresponding to the bit line 19) are connected to onlythe metal plug on the drain. Incidentally, since the metal plug 18formed on the metal plug 29 placed on the drain 10 is substantiallyformed in the interlayer insulating film 17 in the same shape andposition as the corresponding metal plug 29, the illustration thereofwill be omitted to make it easy to understand the drawing. The metalplug 29 (corresponding to the metal plug 16) formed on the source 11 isconfigured so as to extend in the same direction as the direction inwhich the control gate 23 (corresponding to the control gate 3) and thememory gate 27 (corresponding to the memory gate 8) extend, andconstitutes a common source line.

FIG. 4 illustrates the layout of processing mask patterns for formingthe memory gates 8 and 27 only at the side face portions of the controlgates 3 and 23 on the drain 10 side in the memory cell of the presentinvention, as shown in FIGS. 1 and 2. In FIG. 4, reference numerals 191indicate active region patterns for defining active regions surroundedby device isolation regions of the memory cell. The active regions 22are formed so as to extend in the first direction (transverse directionas viewed in the drawing). Reference numeral 192 indicates a first gatefilm pattern for defining an end of the control gate on the drain side,and reference numerals 193 indicate second gate patterns for definingsecond gate films in the process of forming side spacers in order toperform electrode withdrawal of the memory gates 8 and 27, respectively.Further, gate film isolation patterns 194 for cutting off the first andsecond gate films to define an end thereof on the source side andcompleting control gates 199 (corresponding to the control gates 3 and23) and memory gates 200 (corresponding to the memory gates 8 and 27)are shown in FIG. 4. Namely, diagonally-shaded portions of the firstgate film pattern 192 are respectively formed as the control gates 199(corresponding to the control gates 3 and 23) by virtue of the gate filmisolation patterns 194. Of the second gate film patterns 193, portionsshown in high-density patterns are respectively formed as the memorygates 200 (corresponding to the memory gates 8 and 27). Furthermore,contact hole patterns 195 on the memory gates 200, contact hole patterns196 on the control gates 199, drain contact hole patterns 197, andslit-shaped contract hole patterns 198 on the source are shown in FIG.4, and connecting holes 195, 196, 197 and 198 are respectively definedtherein. Incidentally, the metal plugs 16 and 29 are formed within theircorresponding contact hole patterns 198, and the common source lineextending in the second direction (longitudinal direction as viewed inthe drawing) is formed integrally with the metal plugs 16 and 29.Although not shown in the drawing, bit line patterns are disposed inparallel to the active region patterns, and the bit lines 19 and 30 areformed so as to extend in the first direction (transverse direction asviewed in the drawing).

Incidentally, electrode withdrawal portions of the memory gates 8, 27and 200 are electrically connected to their corresponding wirings or viawirings formed in the same layers as the bit lines 19 and 30 through themetal plugs 16 and 29 each formed in the connecting hole 195 of theinterlayer insulating film 15 and the metal plug 18 formed in thecorresponding connecting hole 195 of the interlayer insulating film 17.Further, electrode withdrawal portions of the control gates 3, 23 and199 are electrically connected to their corresponding wirings or viawirings formed in the same layers as the bit lines 19 and 30 through themetal plugs 16 and 29 each formed in the connecting hole 196 of theinterlayer insulating film 15 and the metal plug 18 formed in thecorresponding connecting hole 196 of the interlayer insulating film 17.

As will be described later, in the manufacturing process of the memorycell of the present invention using the mask patterns shown in FIG. 4,device isolation regions 32 for defining the active regions 22 areformed within the substrate 1 by the active region patterns 191.Thereafter, the gate insulating film 2 for the logic transistor (powervoltage MOS transistor) and the memory transistor operated at the powervoltage is grown on the substrate 1, and the first gate film (firstconductive film) formed of, for example, a silicon film is deposited onthe gate insulating film 2. The first gate is thereafterpattern-processed to the shape of the first gate film pattern 192 byusing, for example, a resist film pattern corresponding to the shape ofthe first gate film pattern 192. Afterwards, the gate insulating film 2other than a portion below the first gate film, for example, is removed,and the lower oxide film 5, the silicon nitride films 6 and 25, thelaminated film of the upper oxide films 7 and 26, and the second gatefilm (second conductive film) made of, for example, a silicon, all ofwhich are shown in FIGS. 1 and 2, are deposited on the substrate 1including an upper portion of the first gate film. Incidentally, theupper silicon oxide films 7 and 26 are formed so as to be thicker thanthe thickness of the lower silicon oxide film 5. Thereafter, a resistfilm pattern corresponding to the shape of the second gate film pattern193, for example, is formed, and the second gate film is processed by ananisotropic dry etching method to form side spacer-like second gatefilms on the periphery of the first gate film. Afterwards, the first andsecond gate films are pattern-processed using resist film patternscorresponding to the shapes of the gate film isolation patterns 194, forexample, whereby the processing of the control gates 2, 23 and 199 andthe memory gates 8, 27 and 200 is completed by cutting off the first andsecond gate films. Thereafter, a semiconductor device mixed with a flashmemory is completed via the process of forming the metal wirings 19 and30 after the formation of the source-drain regions 10 and 11 of thememory cell, the formation of source-drain regions of the logictransistor operated at the power voltage, the formation of the metalsilicide films 14, the formation of the interlayer insulating film 15,the formation of the connecting holes 195, 196, 197 and 198, theformation of the interlayer insulating film 17, and the formation of theconnecting holes 195, 196 and 197. Incidentally, although not shown inFIG. 4, for example, the slit-shaped contact hole patterns 198 arerespectively formed so as to extend in positions lying downstream fromthe contact hole patterns 196 as viewed in the second direction(longitudinal direction as viewed in the drawing), where they areelectrically connected to their corresponding wirings or via wiringsformed in the same layers as the bit lines 19 and 30 through the metalplugs formed in the connecting holes of the unillustrated interlayerinsulating film 17.

Basic operations of the memory cell of the present invention are shownin FIGS. 5, 6 and 7. VD indicates a drain voltage, VS indicates a sourcevoltage, and VCG indicates a control gate voltage. VMG indicates amemory gate voltage.

FIG. 5 illustrates a voltage-applied state at the time of an eraseoperation. In the case of an erase operation, a suitable positivevoltage (e.g., VMG=10V) is applied to only the memory gate 8, and otherterminals are all set to 0V (ground potential) corresponding to areference voltage. The erase operation serves so as to inject electronsfrom the semiconductor substrate (well region) 1 side by aFowler-Nordheim (FN) tunneling current flowing through the lower oxidefilm 5 directly below the memory gate 8 and trap them into the siliconnitride film 6 to thereby increase a threshold voltage measured from thememory gate 8 (e.g., VTE=2V). Namely, the electrons are injected intothe silicon nitride film 6 corresponding to the charge storage regionfrom the semiconductor substrate 1 side under tunneling of the electronscaused to pass through the lower oxide film 5 corresponding to the gateinsulating film to thereby trap the electrons into a trap in the siliconnitride film 6. Thus, since the tunneling current-based electrons areinjected via the lower oxide film 5 directly below the memory gate 8,the electrons are trapped into only the silicon nitride film 6 directlybelow the memory gate 8, and, hence, the trapping of the electrons intoeach corner, which has been indicated as the second problem of theconventional memory cell, does not occur. As a result, the problem oferase time degradation caused by the electrons trapped into each cornerof the silicon nitride film 6 at a rewrite operation is solved. In thecase of such an erase operation, the high voltage is applied to only thememory gate 8, and no high voltage is applied to the gate oxide film 2of the read transistor section. The erase time depends on an erasevoltage applied to the memory gate 8, and an effective field intensitydetermined by the ratio between the thickness of the lower oxide filmand an effective oxide-film thickness of a lower oxide film/siliconnitride film/upper oxide film. When, for example, the thickness of thelower oxide film 5 is set to 3 nm, the thickness of the silicon nitridefilm 6 is set to 5 nm and the thickness of the upper oxide film 7 is setto 5 nm, the effective oxide-film thickness of the three layer filmsresults in 10.5 nm. Therefore, the erase voltage to be applied to thememory gate 8 reaches about 10.5V to obtain a field intensity of 10MV/cm at which the FN tunneling current flows into the lower oxide film.Since the upper silicon oxide film 7 is formed so as to be thicker thanthat of the lower silicon oxide film 5, the electrons trapped into thesilicon nitride film 6 can be prevented from being emitted from thesilicon nitride film 6 to the memory gate 8 by tunneling.

FIG. 6 illustrates a voltage-applied state at the time of a writeoperation of the memory cell. In the case of a write operation, a sourceor power voltage Vdd (e.g., VD=1.5V) is applied to the drain 10, asuitable negative voltage (e.g., a voltage equal to twice the powervoltage=−2Vdd=−3V) is applied to the semiconductor substrate (wellregion) 1, and a suitable negative voltage (e.g., −Vdd=−1.5V) is appliedto the control gate 3. In this condition, a suitable negative voltage(e.g., VMG=−7V) is applied to the desired memory gate 8 which is toperform writing, by a write time interval. If the design of the deviceis such that VD−VPW=Vdd−(=2Vdd)=3Vdd reaches the neighborhood of ajunction withstand voltage because the difference in potential betweenthe drain 10 and the semiconductor substrate (well region) 1 is ajunction voltage, then a junction surface portion is forcedly reverseddue to the negative voltage applied to the memory gate 8, so that alarge quantity of hot holes occur beginning with a band-to-bandtunneling phenomenon, and they are thereafter injected into the siliconnitride film due to the negative voltage of the memory gate 8. Namely,the large quantity of hot holes can be injected into the silicon nitridefilm due to the formation of a reverse-direction voltage-applied state(reverse-bias state).

Here, a reverse bias voltage between a reverse bias voltage of a pnjunction at the time that the hot holes or the like occur by theband-to-band tunneling, and a reverse bias voltage of a pn junction atthe time that avalanche hot holes greater than those are produced, willbe referred to as a “junction withstand voltage (junction withstand)”.Accordingly, the state of a reverse bias much larger than when the hotholes or the like occur by the band-to-band tunneling, may be grasped ortaken as the state of application of a reverse voltage near or greaterthan the junction withstand voltage. If an attempt is made to define thejunction withstand voltage quantitatively, then a reverse bias voltageat the time that a backward or reverse current of the order of anallowable leakage current that is allowed to flow into a channel of aMIS (Metal Insulate Semiconductor) held in an off state flows through apn junction (also simply called a “junction”), can be defined as thejunction withstand voltage. In the present specification, the junctionwithstand voltage does not mean a junction breakdown voltage.

As described above, the junction withstand voltage can be defined as thereverse bias voltage at the time that the reverse current of the orderof the allowable leakage current that is allowed to flow into thechannel of the off-state MOS transistor flows through the pn junction(also simply called a “junction”). Therefore, when such an allowableleakage current is assumed to be 10 nA, according to the abovedefinition, the device design may be performed in such a manner that aleakage current of 10 nA occurs between the drain 10 and thesemiconductor substrate (well region) 1 by the reverse bias of 3Vdd.Consequently, a large quantity of hot holes occur with the setting ofthe junction voltage corresponding to the difference in potentialbetween the drain 10 and the semiconductor substrate 1 at the time ofthe write operation to the neighborhood of the junction withstandvoltage. The holes are injected into the silicon nitride film by thenegative voltage of the memory gate 8.

If the device design is performed such that the junction withstandvoltage becomes smaller than 3Vdd, then avalanche hot holes occur inlarger quantities and are injected into the silicon nitride film inlarger quantities, so that the time required to inject them can befurther reduced. Namely, a larger quantity of avalanche hot holes occurwith the setting of the junction voltage corresponding to the differencein potential between the drain 10 and the semiconductor substrate 1 atthe time of the write operation to be greater than the junctionwithstand voltage. Thus, the hot holes are injected into the siliconnitride film in larger quantities and hence the injection time can bereduced.

The injected hot holes neutralize the already-trapped electrons andreduce the threshold voltage measured from the memory gate 8 (e.g.,VTP=−2V). Since a drain current necessary for the write operation is ofonly a leakage current at the drain junction, it corresponds to aleakage current value ranging from about 5 μA/bit to 10 μA/bit near thejunction withstand voltage. This is reduced to 1/10 or less as comparedwith 200 μA/bit at the time of writing based on hot electron injectionin the conventional first memory cell. In the case of writing based onhot hole injection, hot hole generating regions locally exist in a drainjunction end at which the concentration of an electric field occurs, andthe distance at which each hot hole is achievable from its generationpoint, is about 50 nm. Therefore, the width of the memory gate 8 is setin such a manner that an effective channel length of the memorytransistor section reaches 50 nm or less. In the case of only the memorytransistor section, the difficulty of stably controlling the initialthreshold voltage and the drawback of increasing the off leakagecurrent, etc. which have turned into the third problem of theconventional memory cell, are similarly non-existent therein. In thememory cell of the present invention, however, the instability of readcharacteristics can be resolved by the provision of the read transistorsection (selection transistor section).

In the case of the present write operation, a high voltage is applied tothe memory gate 8 and the semiconductor substrate (well region) 1, andthe voltage, e.g., −2Vdd applied to the semiconductor substrate (wellregion) 1 even at the maximum is applied to the gate insulating film 2of the read transistor section. However, if a suitable negative voltage(e.g., VCG=−Vdd) is applied to the control gate 3, then the voltage tobe applied to the gate insulating film 2 reaches Vdd. As a result, thethickness of the gate insulating film 2 can be designed so as to be thinequivalent to the gate oxide film of the logic transistor (power voltageMOS transistor) operated at the power voltage. Accordingly, thereduction in the drain current at the time of reading, which has becomethe first problem of the conventional memory cell, can be resolved.Since the maximum voltage applied to each of the control gate 3 and thedrain 10 is of the power voltage (Vdd), each of the read circuits, suchas a word driver circuit connected to the control gate 3, a senseamplifier circuit connected to the drain 10, etc. can be made of aperipheral transistor (power voltage MOS transistor) operated at a powervoltage, having a gate insulating film having the same film thickness asthe gate insulating film 2 and which is capable of realizing high-speedreading. Incidentally, as will be described later, the gate insulatingfilm 2 is configured with a thickness of, for example, 2.7 nm and isformed so as to be thinner than the thicknesses of the laminated films5, 6 and 7.

FIG. 7 illustrates a read-operated state of the memory cell of thepresent invention. In the case of a read operation, a power voltage(e.g., VD=Vdd=1.5V) is applied to the drain 10, the power voltage (e.g.,VCG=Vdd=1.5V) is applied even to the control gate 3, and other terminalsare set to 0V. Since the voltage to be applied to the memory gate 8 isalso 0V, the turning off or on of a drain current is determinedaccording to whether the threshold voltage of the memory transistor isin an erase state (VTE=2V) or a write state (VTP=−2V). The problem ofdegradation of a read disturb life due to the voltage application to thememory gate 8, which has become the fourth problem of the conventionalmemory cell, is hence resolved. As a read drain current in the writestate, a large current value is obtained from the fact that, since thethickness of the gate insulating film 2 of the read transistor sectionis equivalent to the logic transistor (power voltage MOS transistor),the current drive capacity is high (Gm is high), and since the effectivechannel length of the memory transistor section is 50 nm or less, theparasitic resistance of such a portion is small. When, for example, theread transistor section is compared with a logic transistor having thesame effective channel width/effective channel length, the readtransistor section is capable of achieving a drain current value up toabout 70% to 80% of that of the logic transistor. As a result, a flashmemory having an ultrafast read speed (e.g., read frequency of 200 MHz)can be mixed into a logic LSI in terms of the fact that the above readcircuit can be made of the peripheral transistor (power voltage MOStransistor) operated at the power voltage, and the read current of thememory cell is large.

<<Data Processor>>

FIG. 8 illustrates a data processor with an on-chip flash memory modulewhich includes a memory cell having the structure illustrated in FIGS. 2and 3. Although not restricted in particular, the data processor 200 isformed on a single semiconductor substrate (semiconductor chip) likemonocrystal silicon by a 0.13 μm semiconductor integrated circuitmanufacturing technology. Although not restricted in particular, a largenumber of bonding pads are disposed around the semiconductor substrate.The data processor 200 includes respective circuit modules of a CPU(Central Processing Unit) 201 constituted of a logic MOS transistor(power voltage MOS transistor) having a gate insulating film whosethickness is 2.7 nm, which is operated at a source or power voltageVdd=1.2V, an SCI (Serial Communication Interface) 202, an FRT (FreeRunning Timer) 214, a DSP unit 203, a DMAC (Direct Memory AccessController) 204, an FLC (Flash Controller) 205, a UBC (User BreakController) 206 having a debug support function, a CPG (Clock PulseGenerator) 207, a SYSC (System Controller) 208, a BSC (Bus StateController) 215, a RAM (Random Access Memory) 209 whose memory capacityis, for example, 16 kB, and a JTAG211 used in a self-test or the like.Further, the data processor 200 comprises a flash memory (FLSH) 212which is made of, for example, a logic transistor (power voltage MOStransistor) having a gate insulating film whose thickness is 2.7 nm, ahigh-withstand transistor having a gate insulating film whose thicknessis 15 nm, and the memory cell transistor of the present invention, andwhich has a memory capacity of 256 kB, and an I/O (Input/Output) circuit216. Incidentally, the high-withstand transistor is a transistor havinga gate insulating film which is thicker than that of the gate insulatingfilm of the power voltage MOS transistor.

Although not restricted in particular, an external source or powervoltage supplied to an external power terminal of the data processor 200is set to 3V, and the power voltage Vdd (=1.2V) of the logic MOStransistor (power voltage MOS transistor) is formed by stepping down theexternal power voltage. Each of the MOS transistors constituting the I/Ocircuit 216 has a withstand voltage exceeding 3V. Each of thehigh-withstand MOS transistors for the flash memories 212 and 213 has awithstand voltage which causes no gate breakdown with respect to highvoltages necessary upon write and erase operations for the memory cell.

A detailed example of the flash memory 212 is shown in FIG. 9. The flashmemory 212 has a memory cell block in which a large number of memorycells MCs, each as illustrated in FIGS. 2 and 3, are disposed in matrixform. The memory cells MCs are illustrated as being divided into readtransistor units or sections (RTr) and memory transistor sections (MTr).Although not restricted in particular, the large number of memory cellsMCs are respectively configured as NOR type memory cell blocks whereinsource lines SLs are made common, and n bit lines BL1 through BLn, mcontrol gate lines CG1 through CGm and m memory gate lines MG1 throughMGm are provided. Although not restricted in particular, the memory cellblocks share the well region in which the memory cell transistors areformed. In practice, a large number of memory cell blocks are disposedin obverse and reverse directions of the sheet to thereby constitute aflash memory.

The control gate lines CG1 through CGm are driven by a word driver forread 225. The memory gate lines MG1 through MGm, source lines SLs andwell region PW are driven by a word driver and well driver for write226. The selection of the control gate line and the memory gate line tobe driven is performed by an X decoder 227. The bit lines are connectedto a sense latch circuit and column switch circuit 228. The sense latchcircuit is connectable to data buffers 221 and 222 by the correspondingcolumn switch circuit, and a Y decoder 229 effects the selection of itsconnection on the column switch circuit 228. A power circuit 230generates the internal voltages necessary for memory operations.

The flash memory 212 is placed under access control of the FLC 205 whichresponds to access requests made from the CPU 201 and the DMAC. The FLC205 is connected to the flash memory 212 via address lines ADR1 throughADRi, data lines DAT1 through DATj and control lines ACS1 through ACSk.An address input buffer (AIBUF) 220 inputs address signals through theaddress lines. The input address signals are supplied to the X decoder227 and Y decoder 229 through a predecoder 231. The data input buffer(DIBUF) 221 inputs access commands and write data through the data linesDAT1 through DATj. The data output buffer (DOBUF) 222 outputs read datasent from each memory cell. A control circuit 223 inputs strobe signals,such as a read signal, a write signal, a command enable signal, anaddress enable signal, etc. through the control lines ACS1 through ACSkand controls an input/output operation to the outside. Further, thecontrol circuit 223 inputs an access command through the data inputbuffer 221 and controls a memory operation specified by the inputcommand.

In FIG. 9, each of the word driver and well driver 226 and the powercircuit 230 comprises a high-withstand transistor having a gateinsulating film whose thickness is 15 nm, for example. Other elementcircuits are respectively made of a logic MOS transistor (power voltageMOS transistor) having a relatively thin gate insulating film whosethickness is 2.7 nm. For example, an initial threshold voltage of a readtransistor section (RTr) of each memory cell is designed to be 0.5V, aninitial threshold voltage of each memory transistor section (MTr) isdesigned to be −0.5V, and a drain junction withstand voltage is designedto be 3.6V, respectively.

FIG. 10 illustrates a state at the time of an erase operation effectedon the flash memory. Erasing is performed in memory cell block units,i.e., well region units of the memory cells. Namely, for example, anerase voltage of 10V is applied to all the memory gates (MG1 throughMGm) in an erase block for an erase time of 100 ms, and a groundpotential (Vss) 0V is applied to all of the other terminals to therebytrap electrons into the corresponding silicon nitride film by atunneling current flowing via the lower oxide film below each memorygate MG and to increase the on-erase threshold voltage (VTE) of eachmemory transistor section MTr to 1.2V, whereby the erase operation iscompleted.

FIG. 11 illustrates a state at the time of a write operation effected onthe flash memory. For example, −2Vdd (−2.4V) is applied to the wellregion PW in a write block, −1.2V (−Vdd) is applied to all the controlgate lines CG1 through CGm, and −7V is applied to only the correspondingmemory gate lines (e.g., MG2 and MGm) on which writing is effected.Thereafter, 1.2V (Vdd) is applied to the corresponding bit lines (e.g.,BL2 and BLn) on which writing is effected, for a write time of 10 μs,and hot holes generated in the neighborhood of the drain are injectedinto the silicon nitride film to thereby reduce the threshold voltage(VTP) of each memory transistor section MTr to −1.2V, whereby the writeoperation is completed.

FIG. 12 illustrates a state at the time of a read operation effected onthe flash memory. For example, the corresponding bit line (e.g., BL2) onwhich reading is effected, is selected and precharged to 1.2V (Vdd).Thereafter, 1.2V (Vdd) is applied to the selected control gate line(e.g., CG2), and a change in the potential on the bit line BL2 intendedfor reading is detected by the corresponding sense amplifier circuit,whereby the reading of data is carried out. Since, at this time, theread-intended memory cell connected to the bit line BL2 and the controlgate line CG2 is in a write state and the threshold voltage of eachmemory transistor is set to VTP=−1.5V, an on-current of each memory cellis set to about 50 μA. The sense amplifier circuit detects a change inthe current or a change in voltage with its change.

FIG. 13 illustrates another bit line structure of the memory cell block.The configuration shown in the drawing is of a structure wherein eachbit line is hierarchized into a global or main bit line GL and a sub bitline SBL, only the sub bit line SBL to which the corresponding memorycell MC to be operated and selected is connected, is selected andconnected to the global bit line GL, and the parasitic capacitance ofthe corresponding bit line associated with the memory cell is apparentlyreduced to thereby realize a high-speed read operation. Since there isno need to apply a high voltage to the bit lines BL and GL even in thecase of writing, as described above, it is not necessary to bring a MOStransistor 233 and its driver (Z driver) 234 for selectively connectingthe sub bit line SBL to the global bit line GL into a high withstandstate. Namely, the structure is made of a MOS transistor (power voltageMOS transistor) having a gate insulating film of a relatively thinthickness of 2.7 nm. Even in regard to this point of view, the Gm of apath for reading memory information is further reduced, and, hence, thespeeding-up by the hierarchized bit-line structure based on theglobal/sub bit lines can be fully made functional.

<<Memory Cell Transistor; Threshold Control>>

FIG. 14 shows another example of the nonvolatile memory cell transistor.The memory cell shown in the drawing represents an example in which thedoping of the control gate and memory gate of the memory cell shown inFIG. 1 with the impurity is changed to thereby obtain a desired initialthreshold voltage under the same channel structure. Namely, the wholesurface of a channel region of a semiconductor substrate (well region) 1is brought into depletion by channel implantation, and a control gate 21and a memory gate 8 are changed in conductivity type to thereby causethreshold voltages of a selection transistor section (read transistorsection) and a memory transistor section to differ from each other.

More specifically, according to the vertical sectional structureillustrated in FIG. 14, a read transistor section is formed with thecontrol gate (CG) 21 made of a 150 nm-thick p-type polysilicon filmdoped with a boron whose concentration is 2×10²⁰ cm⁻³, in a surfaceregion of a p-type semiconductor substrate (well region) 1 having aresistivity of 10 Ωcm with a gate insulating film 2 formed of a siliconoxide film having a thickness of 2.7 nm being interposed therebetween.There is also a memory transistor section in which a lower oxide film 5having a thickness of 3 nm, a silicon nitride film 6 having a thicknessof 5 nm, and an upper oxide film 7 having a thickness of 5 nm arelaminated over the surface region of the p-type semiconductor substrate(well region) 1 on the drain side of the control gate (CG) 21, and amemory gate (MG) 8 having a gate length of 50 nm, which is made of ann-type polysilicon film which is doped with phosphor whose concentrationis 4×10²⁰ cm⁻³, and which has a thickness of 150 nm, is formed over theabove films. Incidentally, the memory gate (MG) 8 and the control gate(CG) 21 are electrically isolated from each other by the laminated filmformed of the respective films 5, 6 and 7.

A drain region 10 having a maximum arsenic concentration of 1.5×10²⁰cm⁻³, a junction depth of 40 nm and a junction withstand voltage of 4.5Vis formed in the surface region of the semiconductor substrate (wellregion) which overlaps with the memory gate (MG) 8. A source region 11having a maximum arsenic concentration of 1.5×10²⁰ cm⁻³, a junctiondepth of 40 nm, and a junction withstand voltage of 4.5V is formed inthe surface region of the semiconductor substrate (well region) 1, whichoverlaps with the control gate (CG) 21. Namely, the read transistorsection and the memory transistor section are configured over a channelregion 20 lying between the drain region 10 and the source region 11.

Initial threshold voltages of the read transistor section and the memorytransistor section of the memory cell illustrated in FIG. 14 aredetermined by the n-type channel region 20 formed in the surface regionof the semiconductor substrate (well region) 1. The n-type channelregion 20 is set such that the threshold voltage of the read transistorsection, which is made of, for example, a control gate (CG) 21 formed ofa polysilicon film of conductivity type corresponding to a p type,becomes 0.5V. The average arsenic concentration thereof is 5×10¹⁷ cm⁻³,and the junction depth thereof is 30 nm. At this time, the initialthreshold voltage of the memory transistor section comprising the memorygate (MG) 8 formed of the polysilicon film whose conductivity type is ann type, was −0.5V. Thus, according to the memory cell of the presentembodiment, the initial threshold voltages of the read transistorsection and the memory transistor section can be made appropriate owingto only the formation of the n-type channel region 20.

Write and erase operations effected on the memory cell according to thepresent embodiment are basically similar to the operations of the memorycell shown in FIG. 1. In the case of the erase operation, 10V is appliedto only the memory gate (MG) 10 to inject electrons from thesemiconductor substrate 1 side by a tunneling current and trap them intothe silicon nitride film 6, whereby the memory cell is brought to a highthreshold voltage state. In the case of the write operation, 1.2V (Vdd)is applied to the drain 10, −2.4V (−2Vdd) is applied to thesemiconductor substrate 1, −1.2V (−Vdd) is applied to the control gate(CG) 21, and −7V is applied to the memory gate 8 to inject hot holesgenerated in the neighborhood of a junction surface of the drain 10 intothe silicon nitride film 6, thereby neutralizing trapped electrons,whereby the memory cell is brought to a low threshold voltage state.

<<Manufacturing Method>>

A manufacturing process for mixing the nonvolatile memory cell into alogic LSI by use of a 0.13 μm process technology, for example, will bedescribed using cross-sectional views of an LSI (see FIGS. 15 through30) representing sequential manufacturing process steps. Although notrestricted in particular in the description made herein, the maskpattern layout shown in FIG. 4 will be used for mask patterns forprocessing a memory cell. Incidentally, the left sides of thecross-sectional views (see FIG. 15 through 30) respectively indicate amemory cell forming region (memory cell), the central portions thereofrespectively indicate a power voltage MOS transistor forming region(power voltage MSO), and the right sides thereof respectively indicate ahigh voltage MOS transistor forming region (high voltage MOAS).Incidentally, lines X-X shown in FIG. 15, etc. indicate cut regions ofportions formed by cutting off the left and right portions and plottingthem for convenience.

As shown in FIG. 15, trenches each having a depth of about 250 nm aredefined in a surface region of a p-type semiconductor substrate 31(corresponding to the semiconductor substrate (well region) 1) having aresistivity of 10 Ωcm, for example, and thereafter an oxide film isdeposited. Next, the oxide film is polished by a CMP (ChemicalMechanical Polishing) method to embed the oxide film into the trenches,followed by formation of trench type device isolation regions 32planarized by the CMP method. Thereafter, a surface oxide film 33 havinga thickness of 10 nm is grown on the trench type device isolationregions 32. Incidentally, while the trench type device isolation regions32 are formed so as to define active regions 22, dummy active regionsmay be formed in the trench type device isolation regions to facilitatethe embedding by the CMP method.

Next, as shown in FIG. 16, phosphor ions each having an accelerationenergy of 1 MeV are injected into a desired region in an injectionamount of 1×10¹³/cm² through the surface oxide film 33, and phosphorions each having an acceleration energy of 500 keV are injected thereinin an injection amount of 3×10¹²/cm² therethrough to thereby form ann-type embedding region 34. Thereafter, phosphor ions each having anacceleration energy of 150 keV are injected into a region in which ahigh voltage PMOS transistor is formed, in an injection amount of1×10¹²/cm², thereby forming a high voltage n-type well region 35. Usinga resist pattern 36 having a thickness of 3 μm as a mask in a state ofonly a memory cell region and a high voltage NMOS transistor formingregion being open, boron ions each having an acceleration energy of 500keV are further injected in an injection amount of 1×10¹³/cm², boronions each having an acceleration energy of 150 keV are injected in aninjection amount of 5×10¹²/cm², and boron ions 37 each having anacceleration energy of 50 keV are injected in an injection amount of1×10¹²/cm² to thereby form a high voltage p-type well region 38.

Next, as shown in FIG. 17, phosphor ions each having an accelerationenergy of 100 keV are injected into a region in which powervoltage-operated PMOS transistor is formed, in an injection amount of1×10¹²/cm². Further, phosphor ions each having an acceleration energy of40 keV are injected therein in an injection amount of 5×10¹¹/cm² tothereby form a power voltage n-type well region 39. Using a resistpattern 40 having a thickness of 3 μm as a mask in a state of only aregion in which a power voltage-operated NMOS transistor is formed,being open, boron ions each having an acceleration energy of 200 keV areinjected in an injection amount of 1×10¹³/cm², boron ions each having anacceleration energy of 100 keV are injected in an injection amount of5×10¹²/cm², and boron ions 41 each having an acceleration energy of 30keV are injected in an injection amount of 2×10¹²/cm² to thereby form apower voltage p-type well region 42.

Next, as shown in FIG. 18, boron difluoride (BF₂) ions 44 each having anacceleration energy of 50 keV are injected in an injection amount of2×10¹²/cm² using a resist pattern 43 having a thickness of 1.5 μm as amask in a state of only the memory cell region being open, to therebyform a memory enhance implantation region 45.

As shown in FIG. 19, the resist mask 43 and the surface oxide film 33are thereafter removed. A high voltage gate insulating film 47 having athickness of about 15 nm, which is made of a silicon oxide film, isgrown, by, for example, thermal oxidation, in the region in which thehigh voltage transistor is formed. A power voltage gate insulating film46 (corresponding to the gate insulating film 2) having a thickness ofabout 2.7 nm, which comprises the silicon oxide film, is grown in theregion in which the power voltage-operated transistor (power voltage MOStransistor) is formed, and the region in which the memory cell isformed. Thereafter, they are deposited by chemical vapor deposition(CVD). Further, a non-doped polysilicon film 48 having a thickness of150 nm is deposited. Phosphor ions each having an acceleration energy of5 keV are injected into a region other than the power voltage-operatedPMOS transistor forming region, of the non-doped polysilicon film 48 inan injection amount of 2×10¹⁵/cm² to thereby form an n-type polysiliconfilm 49. A silicon nitride film 50 having a thickness of 100 nm isdeposited thereabove by CVD.

Next, as shown in FIG. 20, the n-type polysilicon film 49 and thesilicon nitride film 50 in the memory cell region are processed usingthe first gate film pattern 192 for defining the drain side of thecontrol gate in the memory cell of the present invention shown in FIG. 4to thereby form first gate film patterns 50 and 51 each corresponding tothe shape of the first gate film pattern 192. Using the first gate filmpatterns as masks, arsenic ions 52 each having an acceleration energy of10 keV are injected in an injection amount of 3×10¹²/cm² to thereby formmemory depletion implantation regions 53. A plan or flat pattern of amemory cell section corresponding to FIG. 20 is shown in FIG. 31.

Incidentally, the polysilicon films 48 and 49 left in the power voltageMOS transistor forming region and the high voltage MOS transistorforming region are respectively configured as gate electrodes of a powervoltage MOS transistor and a high voltage MOS transistor as will bedescribed later. Namely, since it is not necessary to form the gateinsulating film 47 of the high voltage MOS transistor in the subsequentprocess steps, the corresponding memory cell can be formed after theformation of the gate insulating film 47 so as to be thick in thickness.Thus, thermal treatment for forming the thick gate insulating film 47 isnot loaded on the formation of the memory cell, and hence the degree offreedom of device design of the memory cell can be enhanced and a burdenon the forming process can be reduced.

Next, as shown in FIG. 21, a laminated film 54 comprising a lower oxidefilm (corresponding to the lower oxide film 5) of a thermal oxidationfilm having a thickness of about 3 nm, a silicon nitride film(corresponding to the silicon nitride films 6 and 25) having a thicknessof about 5 nm, corresponding to a charge storage region, and an upperoxide film (corresponding to the upper oxide films 7 and 26) of a CVDoxide film having a thickness of about 5 nm is deposited on the surfaceregion of the semiconductor substrate 31 in the memory cell region, forexample. The laminated film 54 in the peripheral transistor region andthe silicon nitride film 50 are removed by dry etching with a resistpattern 55 having a thickness of 2 μm, having covered the memory cellregion alone, being used as a mask. Incidentally, an insulating filmmade of a silicon oxide film 4 is formed on side walls of each firstgate film pattern 51 made of the n-type polysilicon film by thermaloxidation for forming the lower oxide film 5, so as to be thicker thanthe lower oxide film 5.

Next, as shown in FIG. 22, the resist film 55 is removed, and thereaftera non-doped polysilicon film having a thickness of about 50 nm isdeposited over the whole surface of the substrate including thepolysilicon films 48 and 49 by, for example, CVD. Boron difluoride (BF₂)ions each having an acceleration energy of 15 keV are injected into aregion in which a power voltage-operated PMOS transistor at theperipheral portion is formed, in an injection amount of 5×10¹⁵/cm² tothereby form a p-type polysilicon film 57. Phosphor ions each having anacceleration energy of 5 keV are injected into all regions other thanthe power voltage-operated PMOS transistor forming region in aninjection amount of 5×10¹⁵/cm² to thereby form an n-type polysiliconfilm 56.

Next, as shown in FIG. 23, for example, the n-type polysilicon film 56and the p-type polysilicon film 57 are etched by anisotropic dry etchingusing the gate electrode patterns of the peripheral transistors tothereby form a power voltage-operated PMOS transistor gate 61, a powervoltage-operated NMOS transistor gate 58, a high voltage PMOS transistorgate 59 and a high voltage NMOS transistor gate 60. At this time, thememory cell section is simultaneously etched using the second gate filmpatterns 193 shown in FIG. 4, to thereby form contact withdrawal regions193 in regions covered with the second gate film patterns 193 and formside spacer-like memory gates 62 on side walls of the first gate filmpatterns 50 and 51 in regions uncovered with the second gate filmpatterns 193 through the insulating film 4, silicon nitride film 6 andCVD oxide film 7 on a self-alignment basis with respect to the firstgate film patterns 50 and 51. A flat pattern of the memory cell sectionis shown in FIG. 32. Regions surrounded by thick lines 193 are coveredwith resist patterns and serve as the contact withdrawal regions 193.Portions uncovered with resist patterns serve as the sidewall spacers 62and are formed on the side walls of the first gate film patterns 50 and51 each corresponding to the shape of the first gate film pattern 192.

Next, as shown in FIG. 24, the silicon nitride films 50 on the firstgate film patterns 51 are removed by dry etching with, for example, a2-μm thick resist film 63 being used as a mask in a state of the memorycell region being open. Thereafter, arsenic ions 64 each having anacceleration energy of 20 keV are injected in an injection amount of5×10¹⁴/cm² with the resist film 63 being used as the mask to therebyform memory drains 65. As shown in FIG. 24, vertical intervals arerespectively formed between the side space-like memory gates 62 andcontrol gates based on the first gate film patterns 51. Namely, eachside spacer-like memory gate 62 is formed higher than that of thecontrol gate based on each first gate film pattern 51.

Next, as shown in FIG. 25, the first gate film patterns 51 are cut bypatterning according to dry etching using a 0.8-μm thick resist film 66formed to etch the shapes of the gate film isolation patterns 194 of thememory cell shown in FIG. 4 as a mask to thereby pattern-process thecontrol gate of the memory cell. Subsequently, arsenic ions 67 eachhaving an acceleration energy of 20 keV are injected in an injectionamount of 5×10¹⁴/cm² using the resist film 66 as a mask to thereby forma source (region) 68 of the memory cell. A flat pattern of the memorycell section corresponding to FIG. 25 is shown in FIG. 33. When theportions indicated by the gate film isolation patterns 194 of theportions indicated by the first gate film pattern 192, contactwithdrawal regions 193 and memory gates 62 are removed by patterning,regions designated at 199 are left in the region of the first gate filmpattern 192, so that the control gates 51 (199, 2 and 23) of theirmemory cells are formed. The regions indicated by the contact withdrawalregions 193 and memory gates 62 are formed on the side walls of thecontrol gates 51 (199, 2 and 23) and are respectively separated fromeach other to form memory gates 62 (8, 27 and 200) of the respectivememory cells.

Next, as shown in FIG. 26, boron difluoride ions each having anacceleration energy of 20 keV are injected into, for example, the powervoltage-operated PMOS transistor section alone in an injection amount of2×10¹⁴/cm², and phosphor ions each having an acceleration energy of 10keV are injected therein in an injection amount of 3×10¹³/cm² to therebyform a p-type extension 70. Arsenic ions each having an accelerationenergy of 10 keV are injected into the power voltage-operated NMOStransistor section alone in an injection amount of 2×10¹⁴/cm², and boronions each having an acceleration energy of 10 keV are injected thereinin an injection amount of 2×10¹³/cm² to thereby form an n-type extension71. Boron ions each having an acceleration energy of 20 keV are injectedinto the high voltage PMOS transistor section alone in an injectionamount of 1×10¹³/cm² to thereby form a low-concentration p-typesource/drain 72. Phosphor ions each having an acceleration energy of 30keV are injected into the high voltage NMOS transistor section alone inan injection amount of 2×10¹³/cm² to thereby form a low-concentrationn-type source/drain 73. Thereafter, they are deposited by CVD, and oxidefilm side spacers 69 each corresponding to a 75-nm thick insulating filmprocessed by an etchback method using on anisotropic etching arerespectively formed on both side walls of the memory gates 62 (8, 27 and200) and the side walls of the control gates 51 (199, 2 and 23) on aself-alignment basis. The oxide film side spacer 69 formed on one sidewall of each of the memory gates 62 (8, 27 and 200) is formed on itscorresponding control gate 51 (199, 2 and 23), whereas the oxide filmside spacer 69 formed on the other side wall is formed on the drainregion 65 side. The oxide film side spacers 69 formed on the side wallsof the control gate 51 (199, 2 and 23) are formed on the source region68 side.

Next, as shown in FIG. 27, boron difluoride ions each having anacceleration energy of 20 keV are injected into only the PMOS transistorsections at the peripheral portion, for example, in an injection amountof 3×10¹⁵/cm² to thereby form high-concentration p-type source/drains 90and 75. Arsenic ions each having an acceleration energy of 30 keV areinjected into only the NMOS transistor sections at the peripheralportion in an injection amount of 3×10¹⁵/cm² to thereby formhigh-concentration n-type source/drains 74 and 76. Thereafter, a cobaltsilicide (CoSi) film 77 having a thickness of 40 nm is grown on all thegates 58, 59, 60 and 61, the source/drains 70 through 76 and 90, thegates 51 and 62 of the memory cell, and the source/drains 65 and 68 atthe peripheral portion by using a salicide technology. Further, an oxidefilm 78 having a thickness of about 30 nm and a silicon nitride film 79having a thickness of about 50 nm are deposited by, for example, CVD asan insulating film as shown in FIG. 28. Incidentally, the cobaltsilicide (CoSi) film 77 is formed by, for example, depositing a cobalt(Co) film on the whole area on the main surface of the substrate,thereafter causing cobalt and silicon to react by thermal treatment andsubsequently removing the unreacted cobalt (Co) film. No cobalt issilicidized over the insulating film such as the silicon oxide film orthe like, and the cobalt silicide (CoSi) film 77 is selectively formedon the gates and source/drains formed of silicon. As described above,the vertical interval is formed between each side spacer-like memorygate 62 and its corresponding control gate based on the first gate filmpattern 51, and the insulating film side spacers 69 are formed on theside walls of the memory gates 62 so as to be placed between them.Therefore, there is no possibility that the cobalt silicide film 77 oneach memory gate 62 and the cobalt silicide film 77 on each control gate51 will be shorted. Since the insulating film side spacer 69 is formedon the side wall on the drain 65 side, of each memory gate 62 so as tobe placed between the side spacer-like memory gate 62 and the drain 65,there is no possibility that the cobalt silicide film 77 on the memorygate 62 and the cobalt silicide film 77 on the drain 65 will be shorted.Since the insulating film side spacers 69 are formed on the side wallson the source 68 side, of the control gates 51 so as to be placedbetween the side spacer-like control gates 51 and the source 68, thereis no possibility that the cobalt silicide film 77 on the memory gate 62and the cobalt silicide film 77 on the source 68 will be shorted.

Next, as shown in FIG. 29, for example, an ozone (O₃)-TEOS (siliconoxide film) film 80 having a thickness of about 700 nm is deposited asan interlayer insulating film by CVD. Thereafter, the interlayerinsulating film 80 is polished by CMP to planarize its surface. Next,plug holes (connecting holes) are made open on all the gates andsource/drains to be connected, and, for example, tungsten (W) isembedded into the plug holes to form plugs 81. Common source lines forthe memory cells are connected to one another by the plugs 81.

Finally, as shown in FIG. 30, an interlayer insulating film 82 having athickness of about 300 nm is deposited by CVD, for example. Contactholes (connecting holes) are made open directly above all the plugs 81at the peripheral portion and the plugs 81 on the drains of the memorycell. Contact plugs 83 each formed of tungsten (W) are embedded into thecontact holes in a manner similar to the plugs 81, and a first metalwiring 84 each made of a tungsten film having a thickness of about 200nm is formed, whereby the major manufacturing process of the flashmemory-mixed logic LSI according to the present embodiment is completed.Although not shown in the drawing, the process of adding desired metalwirings by a multilayered wiring structure, the deposition of apassivation film and the opening of bonding holes are carried out, andthe initial to final processes are completed.

According to the above-described example illustrative of themanufacturing method of the present invention, a gate length of thelogic transistor (power voltage MOS transistor) at the peripheralportion was 100 nm, a gate length of the high voltage transistor was 0.5μm, a control gate length of each memory cell was 150 nm, a memory gatelength was 50 nm, a memory channel width was 180 nm, a bit line pitchwas 0.3 μm, a word line pitch was 0.5 μm, and the area of the memorycell was 0.15 μm². As a read current for the memory cell, about 50μA/cell can be achieved at a power-voltage 1.2V operation.

<<Another Manufacturing Method>>

A description will next be made of a manufacturing method in which, inthe manufacturing process of mixing the nonvolatile memory cell into thelogic LSI by the 0.13 μm process technology as described above, a memorycell is adopted in which the electrode structure thereof is partlychanged. The basic process of the manufacturing method in this case isalmost the same as that described with reference to FIGS. 15 to 29.Changes thereof will be described using FIG. 34.

As shown in FIG. 34, a common source line of the memory cell is used asa first metal wiring 85 made of an aluminum film having a thickness ofabout 400 nm and configured in common with a first metal wiring 85 ofeach transistor at a peripheral portion. An interlayer insulating film86 whose surface is planarized by CMP, is formed over the first metalwring 85, and contact plugs 87 each made of tungsten (W) are formed inthe interlayer insulating film 86. The contact plugs 87 are directlyconnected to plugs 81 disposed directly above on a drain of the memorycell, and a second metal wiring 88 formed of an aluminum film having athickness of about 400 nm, which is used as a bit line thereabove, isconfigured in common with a second metal wiring 88 of each transistor atthe peripheral portion. The interlayer insulating film 86 with thecontact plugs 87 defined therein is about 700 nm in thickness. Formingthe common source line and the wirings for connecting between thetransistors at the peripheral portion by using the first metal wiring 85formed of the aluminum film in this way makes it possible to reducewiring resistances and enhance the operating speed.

<<Further Manufacturing Method>>

A description will be made here of a method for processing both controland memory gates in the memory cell of the present invention on aself-alignment basis without depending on processing by lithography. Themethod will be described with reference to FIGS. 35 through 39 whichshow sectional structures of a memory cell section as a sequence ofmanufacturing process steps.

FIG. 35 shows a state in which processing is carried out to provide agate oxide film 92 (corresponding to the gate insulating film 2) havinga thickness of 2 nm is grown on a desired memory-cell forming region ofa p-type silicon substrate (well region) 91 having a resistivity of 10Ωcm, for example, a laminated film of a first gate film pattern 93 madeof a silicon film doped with phosphor having a concentration of2×10²⁰/cm³ with a thickness of 100 nm, and a cap nitride film 94 havinga thickness of 200 nm, followed by growth of a lower oxide film 95(corresponding to the lower oxide film 5) having a thickness of 3 nm bya thermal oxidation method, and deposition of a silicon nitride film 96(corresponding to the silicon nitride film 6) having a thickness of 5 nmand an upper oxide film 97 (corresponding to the upper oxide film 7)having a thickness of 5 nm, and side spacer-like memory gates 98(corresponding to the memory gate 8) formed by etching back apolysilicon film doped with phosphor having a concentration of2×10²⁰/cm³ with a thickness of 70 nm are further provided.

Next, as shown in FIG. 36, for example, arsenic ions each having anacceleration energy of 30 keV are injected in an injection amount of4×10¹⁴/cm² into the memory gates 98 from outside to thereby form drains99 (corresponding to the drain 10). Thereafter, the cap nitride film 94is removed by wet etching using the silicon nitride film 96 as a mask,followed by deposition and etchback of an oxide film having a thicknessof 150 nm, whereby oxide film side spacers 100 (corresponding to theoxide film side spacers 12, 13 and 69) each corresponding to aninsulating film having a spacer length of 150 nm is formed.

Next, as shown in FIG. 37, for example, a resist pattern is formed inwhich only a region for the first gate film pattern 93 to be cut is madeopen. The first gate film pattern 93 is processed on a self-alignmentbasis with respect to the oxide film side spacers 100 by dry etchingusing the oxide film side spacers 100 as masks, whereby control gates101 (corresponding to the control gate 3) are formed on a self-alignmentbasis with respect to the oxide film side spacers 100.

As shown in FIG. 38 as well, arsenic ions each having an accelerationenergy of 30 keV are injected into, for example, a region used as thesource between the control gates 101 and 101 in an injection amount of4×10¹⁴/cm² from the vertical direction to thereby form a source 103(corresponding to the source 11). Boron ions each having an accelerationenergy of 20 keV are injected in an injection amount of 2×10¹³/cm² fromthe direction diagonally angled at 30° to thereby form a p-type hollowregion 102 having an impurity concentration higher than an impurityconcentration of a channel region. At this time, the completed controlgate length is 130 nm, and the top of each memory gate 98 is etched 120nm, so that its height reaches 150 nm.

Finally, as shown in FIG. 39, an insulating film 104 having a thicknessof 700 nm is deposited, so that tungsten plugs 105 for connectingopenings of plug holes and a common source line are embedded into theplug holes. A contact interlayer film 106 having a thickness of 300 nmis deposited, so that contact plugs 107 formed of tungsten are embeddedinto contact holes through their openings, followed by formation of abit line 108 made of a tungsten film having a thickness of 300 nm,whereby a major portion of the memory cell is completed.

While the gate length of each control gate 101 is 120 nm and the gatelength of each memory gate 98 is 60 nm in the memory cell manufacturedby the present method, both the gate lengths are also determined by sidespacer lengths (the width of each oxide film side spacer 100 as viewedin a channel-length direction and the width of each side spacer-likememory gate as viewed in a channel-length direction) processed with thefilm thickness deposited by CVD as the reference. Variations in the gatelength within a wafer surface were within ±10%, i.e., the gate length ofeach control gate 101 was 120±12 nm, and the gate length of each memorygate 98 was 60±6 nm. Since the alignment accuracy of the lithographytechnology under the 0.13-μm process technology is about ±30 nm, thevariations in the gate lengths are difficult to attain, so that thevalidity of the present embodiment was confirmed.

FIGS. 40 through 43 respectively show an example in which a tungstenpolycide (WSi₂/poly Si) film is applied to the control gates 101. Withrespect to FIG. 35, for example, the first gate film pattern 93 can bechanged from the polysilicon film (poly Si) to a structure whereinsilicide, like tungsten silicide (WSi), is provided on the poly Si, or ametal gate structure formed of a metal film, as shown in FIG. 40, isprovided. Incidentally, the first gate film pattern 93 is not limited tosilicide, but may be configured as a polymetal structure in which ametal such as W or the like is provided on poly Si with a barrier metalfilm such as WN interposed therebetween. A silicide film such as acobalt silicide (CoSi₂) film may be formed on each memory gate 98 byusing the salicide technology. In this case, the process cross-sectionof FIG. 36 is represented as shown in FIG. 41, and the processcross-section of FIG. 37 is represented as shown in FIG. 42. Thus, thewiring resistance of the control gate 101 can be reduced as comparedwith the formation of each control gate 101 by the silicon film, andhence an increase in operating speed can be achieved. Forming the cobaltsilicide (CoSi₂) on the memory gate 98 makes it possible to reduce thewiring resistance of the memory gate 98 and achieve an increase inoperating speed.

A modification of salicide is shown in FIGS. 43 and 44. Subsequent tothe process of FIG. 37, oxide film (SiO₂) side walls each correspondingto an insulating film are formed on their corresponding side walls ofthe control gates 101 on a self-alignment basis as shown in FIG. 43.Thereafter, a CoSi salicide layer may be formed on the diffusion layerscorresponding to the source/drains 99 and 103 and the memory gates 98 bythe salicide technology. Incidentally, the oxide film (SiO₂) side wallseach corresponding to the insulating film are thereafter formed on theircorresponding side walls of the control gates 101 on a self-alignmentbasis even in the case of FIG. 42 as illustrated in FIG. 44. Afterwards,the CoSi salicide layer may be formed on the diffusion layerscorresponding to the source/drains 99 and 103 by the salicidetechnology. Forming the SiO₂ side walls on their corresponding sidewalls of the control gates 101 on a self-alignment basis makes itpossible to electrically isolate the drain 103 and the CoSi salicidelayer and reduce the resistances of the source/drains 99 and 103 and thewiring resistances of the memory gates 98, thereby enabling an increasein operating speed.

<<Multi-Valued Memory Cell>>

Next, an example of application of a 2-bit/cell having a virtual groundarray configuration to a so-called multi-valued memory cell will bedescribed.

A flat or plane layout of the multi-valued memory cell is illustrated inFIG. 45. In FIG. 45, reference numerals 110 indicate zigzag-shapedactive regions surrounded by device isolation regions, referencenumerals 111 indicate control gates (each corresponding to the controlgate 3), and reference numerals 115 indicate data lines each formed of ametal wiring, which are disposed in the direction normal to the controlgates 111. A laminated film 112 comprising a lower oxide film(corresponding to the lower oxide film 5), a silicon nitride film(corresponding to the silicon nitride film 6), and an upper oxide film(corresponding to the lower oxide film 5) is formed below each memorygate 113 (corresponding to the memory gate 8). The memory gates 113 aredisposed on their corresponding side walls of the control gates 111 withthe laminated films 112 respectively interposed therebetween. Metalplugs 114 for connecting the active regions and the data lines 115 aredisposed at the corners of the zigzag-shaped active regions 110. Thelayout pitch of each data line 115 is designed to be twice (2F) theminimum processing size F, the layout pitch of each control gate 111 isdesigned to be 4F, and the physical cell area is 8F². Thus, since thelayout angle θ of each zigzag-shaped active region 110 to the data line115 is tan θ=(data line pitch)/(control gate pitch)=2F/4F=0.5, θ resultsin about 26.6°

A plane layout of the contact withdrawal portions to the control gates111 and memory gates 113 is illustrated in FIG. 46. Before theprocessing of the memory gates 113 formed in the side spacer are formedby etchback based on the anisotropic dry etching, a resist pattern, towhich a second gate processing pattern 116 is transferred, is disposedat the ends of the control gates 111 to perform etching. In order toindependently withdraw or take out the memory gates 113 at both sideportions of the control gates 111, a polysilicon film processed to theshape of the second gate processing pattern 116 is nextpattern-processed using resist films, to which isolation hole patterns117 (diagonally-shaded portions) are transferred, as masks to therebytake out the memory gates 113 through contact holes 114 and first metalwirings 118 for the control gates. While, at this time, the withdrawalportions of the control gates 111 are connected to the contact holes 114by first metal wirings 119 for the control gates, the side spacer-shapedmemory gates 113 are cut off by the isolation hole patterns 117(diagonally-shaded portions) even at the ends of the control gates 111at such portions. Thus, the second gate processing pattern 116 and theisolation hole patterns 117 (diagonally-shaded portions) of the sidespacer-shaped memory gates 113 are removed, so that the memory gates 113at both side portions of the control gates 111 are independently formed.The layout pitch of each of the first metal wirings 118 for the memorygates is twice (2F) the minimum processing size F, the layout pitch ofeach of the first metal wirings 119 for the control gates is 4F, and thelayout pitch of each of the data lines 115 is 2F, respectively. Aprocessing technology of F=0.2 μm is applied to the memory cellaccording to the present embodiment. A physical memory cell area is2F×4F=0.4×0.8 μm²=0.32 μm². Since the memory cell is 2bit/cell-operated, an effective cell area is 0.16 μm².

A vertical cross-section of the multi-valued memory cell is illustratedin FIG. 47. In the multi-value memory cell, a control gate 123 having agate length of 200 nm, which is made of a polysilicon film doped withphosphor having a concentration of 2×10²⁰/cm³ with a thickness of 200nm, is disposed over the surface of a p-type well region 121 formed in asurface region of a p-type silicon substrate having a resistivity of 10Ωcm with a gate oxide film 122 (corresponding to the gate insulatingfilm 2) having a thickness of 4.5 nm being interposed therebetween.Lower oxide films 124 each having a thickness of 3 nm, silicon nitridefilms 125 each having a thickness of 5 nm and upper oxide films 126 eachhaving a thickness of 5 nm are laminated on the surface regions of thep-type well on the left and right sides of the control gate 123. Sidespacer-shaped memory gates 127 made of a polysilicon film doped withphosphor having a concentration of 2×10²⁰/cm³ with a thickness of 70 nmare respectively disposed over the laminated films. Arsenic ions eachhaving an acceleration of 30 keV are vertically injected from outsidethe memory gates 127 in an injection amount of 4×10¹⁴/cm² to formsource/drain electrodes (memory electrode in which one thereof serves asa source electrode and the other serves as a drain electrode) having ajunction withstand voltage of 5V. The left source/drain electrode 128 isalso called a left source/drain SDL, and the right source/drainelectrode 128 is also called a right source/drain SDR. The gateelectrodes to be controlled, of the multi-valued memory cell shown inthe drawing, consist of three gates, including the control gate 123(also called control gate CG), the left memory gate 127 (also calledleft memory gate MGL), and the right memory gate 127 (also called rightmemory gate MGR).

In FIG. 47, the multi-valued memory cell is capable of storingfour-value or quaternary information. An erase state (e.g., memoryinformation “00”) is realized by applying 10V to the left memory gateMGL and the right memory gate MGR, injecting electrons from the p-typewell 121 to trap them into the silicon nitride films 125, therebybringing threshold voltages measured from the memory gates 127 to 1.5V.A first write state (e.g., memory information “10”) is realized, asillustrated in FIG. 47, by applying 5V to the left source/drain SDL,applying −8V to the left memory gate MGL, injecting hot holes into onlythe left silicon nitride film 125, thereby bringing a threshold voltagemeasured from the left memory gate MGL to −1.5V. Although not shown inthe drawing, a second write state (e.g., memory information “01”) isrealized by applying 5V to the right source/drain SDR, applying −8V tothe right memory gate MGR, injecting hot holes into only the rightsilicon nitride films 125, thereby bringing a threshold voltage measuredfrom the right memory gate MGR to −1.5V. Although not shown in thedrawing, a third write state (e.g., memory information “11”) is realizedby performing a write operation for obtaining a first write state and awrite operation for obtaining a second write state.

A memory array in which multi-value memory cells are disposed in matrixform, is illustrated in FIG. 48. 12 memory cells are typically disposedin the memory array in matrix form. CG1 through CG4 aretypically-illustrated control gate lines, MG1L through MG4L are leftmemory gate lines, MG1R through MG4R are right memory gate lines, andDL1 through DL4 are data lines, respectively. The data lines arerespectively shared between right source/drain SDRs and leftsource/drain SDLs of the adjacent memory cells.

An erase operation of each memory cell will be described with referenceto FIG. 48. All of the left and right memory gates MG1L through MGL4Land MG1R through MG4R in an erase block are selected. 10V is applied tothem for an interval corresponding to an erase time of 100 ms andelectrons are injected therein by tunneling currents and are trappedinto the silicon nitride films 125 as shown in FIG. 47. A thresholdvoltage measured from each memory gate is set to VTE=1.5V.

Now, an erase state, a write state, and threshold voltage states of leftand right memory gates in one memory cell will be described as “0”, “1”and “L, R” (L, R=“0” or “1”) respectively. After the erase operation,all the memory cells are understood or taken as states of storing erasedata “0, 0”.

FIG. 49 illustrates a write operation. −8V is applied to selected memorygates to be written, e.g., MG1R, MG2L, MG3R and MG4L. Thereafter, 5Vcorresponding to a source/drain junction withstand voltage is applied tothe selected data line DL2 for an interval corresponding to a write timeof 10 μs to thereby inject hot holes based on band-to-band tunnelingcurrents produced in source/drain junction surfaces into the siliconnitride films 125 already placed under electron traps to neutralize theelectron traps, and reduce threshold voltages measured from the memorygates to VTP=−1.5V, whereby the write operation is completed. In thepresent write state, memory cells MCa and MCb respectively store data“0, 1”, and memory cells MCc and MCd respectively store data “1, 0”.

In the case of a write operation, a data disturb voltage of 5V isapplied to only the source/drain of each write-nonselected memory cellnot subjected to writing, or a word disturb voltage of −8V is applied toeach memory gate alone. However, the time necessary for a slightvariation (ΔVTE=0.1V) in threshold voltage due to any one of the disturbvoltages, a so-called disturb life is 1 s or more and includes anoperation margin of 5 digits or more with respect to a write time 10 μs.While 5V corresponding to the source/drain junction withstand voltage isapplied to the selected data line DL2 for the interval corresponding tothe write time of 10 μs in the case of a write operation, 1.8V of apower voltage and −3.2V may respectively be applied to the selected dataline DL2 and the semiconductor substrate to set an effectivesource/drain applied voltage as 5V. Thus, the maximum voltage to beapplied to the data lines and control gates can be set to 1.8V inclusiveof a read operation to be described below. As a result, a word driverconnected to the control gates and a sense amplifier circuit connectedto the data lines can be made of transistors each having a thin-filmgate oxide film operated at the power voltage, whereby high-speedreading is achieved.

The read operation is illustrated in FIGS. 50 and 51. A read operationeffected on one memory cell comprises a read operation for a plusdirection and a read operation for a reverse direction. Theplus-direction read operation is defined as the operation of determiningwhether a current path is formed when one of the left source/drain andthe right source/drain of the memory cell is configured as a drainelectrode. Contrary to the above, the reverse-direction read operationis defined as the operation of determining whether a current path isformed when the other of the left source/drain and the rightsource/drain of the memory cell is configured as a drain electrode.

FIG. 50 illustrates the read operation for plus direction. The drawingillustrates a case in which a memory cell MCc having data “1, 0” writtentherein is intended for reading. In FIG. 50, the data line DL2 and thedata line DL1 ranking ahead of it are precharged to the power voltage1.8V and the control gate CG2 is thereafter raised to the power voltage1.8V, whereby a change in the potential of the data line DL2 is detectedby the corresponding sense amplifier. While the data line DL2 isoperated as the drain and the data line DL3 is operated as the source atthis time, a drain current is cut off because the memory gate MG2R inthe neighborhood of the source is in an erase state, so that thepotential of the data line DL2 remains unchanged. Namely, erase data “0”is read. Reverse-direction reading is subsequently performed. In FIG.51, the data line DL3 and the data line DL4 ranking lower than it areprecharged to the power voltage 1.8V and the control gate CG2 isthereafter raised to the power voltage 1.8V, whereby a change in thepotential of the data line DL3 is detected by the corresponding senseamplifier. While the data line DL3 is operated as the drain and the dataline DL2 is operated as the source contrary to the above at this time, adrain current flows because the memory gate MG2L in the neighborhood ofthe source is in a write state, so that the potential of the data lineDL3 is lowered. Namely, write data “1” is read. Memory cells in whichdata “0, 0”, data “0, 1” and data “1, 1” have been written, can be readaccording to the procedures of similar plus-direction reading andreverse-direction reading.

Although not shown in the drawing in particular, the relationshipbetween the selective control of the data lines, control gate lines andmemory gate lines used upon the write and read operations, and accessaddresses can be arbitrarily determined by the logic of the X and Ydecoders described with reference to FIG. 9. Assuming that byteaddresses are taken, for example, eight memory cells sharing one dataline may be selected with respect to one byte address such that a totalof eight memory transistor sections are intended for writing or reading.The write operation may be effected on the eight memory cells on aparallel basis. As to the read operation, the plus-direction reading andthe reverse-direction reading may be effected on the eight memory cellsin several. If the eight memory cells, whose operations are selected byone byte address, are configured in discrete memory mats or memoryblocks, then read operations for the eight memory cells can be alsoperformed as eight on a parallel basis.

A method of manufacturing the multi-valued memory cell will be describedwith reference to FIGS. 52 through 57.

As illustrated in FIG. 52 by way of example, a trench type deviceisolation region 122 obtained by embedding an oxide film into a trenchhaving a depth of 250 nm and planarizing it by a CMP (ChemicalMechanical Polishing) method is first formed in a surface region of ap-type semiconductor substrate 121 having a resistivity of 10 Ωcm.Thereafter, phosphor ions each having an acceleration energy of 1 MeV,phosphor ions each having an acceleration of 500 keV, and phosphor ionseach having an acceleration energy of 150 keV are respectively injectedinto a desired region through a surface oxide film having a thickness of10 nm in injection amounts of 1×10¹³/cm², 3×10¹²/cm², and 1×10¹²/cm² toform an n-type well region 125. Boron ions each having an accelerationenergy of 500 keV are injected in an injection amount of 1×10¹³/cm², andboron ions each having an acceleration energy of 150 keV are injected inan injection amount of 5×10¹²/cm² to form a high withstand p-type wellregion 124. Boron ions each having an acceleration energy of 500 keV,boron ions each having an acceleration energy of 150 keV, and boron ionseach having an acceleration energy of 50 keV are respectively injectedin injection amounts of 1×10¹³/cm², 5×10¹²/cm² and 1×10¹²/cm² to form ap-type well region 123. Thereafter, boron difluoride (BF₂) ions eachhaving an acceleration energy of 50 keV are injected into a memory cellregion in an injection amount of 7×10¹²/cm² to form a memory channelimplantation region 126. Phosphor ions each having an accelerationenergy of 50 keV are injected into a power voltage-operated PMOStransistor region in an injection amount of 4×10¹²/cm² to form a p-typechannel enhance implantation region 128. Boron difluoride (BF₂) ionseach having an acceleration energy of 50 keV are injected into a highvoltage NMOS transistor region in an injection amount of 3×10¹²/cm² toform an n-type channel enhance implantation region 127. Thereafter, athin-film gate oxide film 129 having a thickness of 4.5 nm is grown onthe memory cells region and power voltage-operated transistor region,and a thick-film gate oxide film having a thickness of 15 nm is grown onthe high voltage transistor region. A non-doped polysilicon film 131having a thickness of 200 nm is deposited by CVD, and phosphor ions eachhaving an acceleration energy of 10 keV are injected into the memorycell region and NOS transistor region in an amount of 4×10¹⁵/cm² to forma first n-type gate film 132. Thereafter, the n-type gate film 132 inthe memory cell region alone is processed to form control gates 133.

Next, as shown in FIG. 53, a lower oxide film 134 having a thickness of3 nm is grown by a thermal oxidation method, and a silicon nitride film135 having a thickness of 5 nm is deposited thereabove by CVD. Further,an upper oxide film 136 having a thickness of 5 nm is deposited andthereafter the lower oxide film 134, silicon nitride film 135 and upperoxide film 136 in a peripheral region other than the memory cell regionare removed.

Next, as shown in FIG. 54, a non-doped polysilicon film having athickness of 50 nm is deposited by CVD, and phosphor ions each having anacceleration energy of 10 keV are injected into the memory cell regionand NMOS transistor region in an injection amount of 2×10¹⁵/cm² to formsecond n-type gate films 137. Boron difluoride (BF₂) ions each having anacceleration energy of 10 keV are injected into the PMOS transistorregion in an injection amount of 5×10¹⁵/cm² to form a p-type gate film138.

Further, as shown in FIG. 55, a laminated film of the first n-type gatefilm and the second n-type gate films, and the p-type gate film areprocessed to form a p-type gate electrode 140 and an n-type gateelectrode 139. In the same gate processing process, the second n-typegate film 137 in the memory cell region is processed into side spacershapes to thereby form memory gates 141 of the memory cell.

Next, as shown in FIG. 56, boron difluoride ions each having anacceleration energy of 20 keV, and phosphor ions each having anacceleration energy of 10 keV are respectively injected into the powervoltage-operated PMOS transistor section alone in injection amounts of2×10¹⁴/cm² and 3×10¹³/cm² to form p-type extensions 142. Phosphor ionseach having an acceleration energy of 30 keV are injected into the highvoltage NMOS transistor section alone in an injection amount of6×10¹²/cm² to form low-concentration n-type source/drains 143. Arsenicions each having an acceleration energy of 10 keV are injected into thememory cell region alone in an injection amount of 5×10¹⁴/cm² to formmemory source/drains 144. Thereafter, oxide film side spacers 145 eachhaving a thickness of 80 nm, which are deposited by CVD and processed byetchback, are formed, and boron difluoride ions each having anacceleration energy of 20 keV are injected into the peripheral PMOStransistor region in an injection amount of 3×10¹⁵/cm² to formhigh-concentration p-type source/drains. Arsenic ions each having anacceleration energy of 30 keV are injected into the peripheral NMOStransistor region in an injection amount of 3×10¹⁵/cm² to formhigh-concentration n-type source/drains. Afterwards, an oxide film 146having a thickness of 30 nm, which is deposited by CVD, is furtherremoved by wet etching with only the memory cell region left behind, andcobalt silicide films 147 each having a thickness of 40 nm are formed onall the gate electrodes and source/drains of the peripheral transistors.

Finally, as illustrated in FIG. 57, a silicon nitride film 148 having athickness of 50 nm is deposited by CVD, and an O₃-TEOS film 149 having athickness of 700 nm is further deposited by CVD. Thereafter, plug holesare defined above all the gates and source/drains to be connected, andtungsten (W) is embedded therein to form plugs 150, and first metalwirings 151 made of a tungsten film having a thickness of 200 nm areformed, whereby the major manufacturing process for the 2-bit/cell flashmemory according to the present embodiment is completed. Further,although not shown in the drawing, the process of adding desired metalwirings, the deposition of a passivation film and the opening of bondingholes are carried out, and the initial to final processes are completed.

While the invention developed by the present inventors has beendescribed specifically based on the illustrated embodiments, the presentinvention is not limited to the embodiments. It is needless to say thatvarious changes can be made thereto within a scope not departing fromthe substance thereof.

For example, the above description has been made of a case in which, asa best mode, a nonvolatile memory cell transistor according to thepresent invention has a configuration so as to perform informationstorage by the injection of hot holes from the drain side and theinjection of electrons from a well region, as one example. However, thepresent invention is not limited to this example in principle. Forexample, combinations of the injection of electrons from the memory gateside, the injection of hot electrons as an alternative to FN tunneling,the injection of hot holes by FN tunneling, and the injection of hotelectrons may be adopted. The concept of writing and erasing is ageneral concept, and a state in which the threshold voltage is high anda state in which the threshold voltage is low, respectively, may bedefined as writing and erasure. Various applied voltages for writing anderasure can be changed in various ways according to the relationshipswith a source or power voltage of an LSI in which the correspondingmemory cell is provided on-chip, the generation of a manufacturingprocess, other on-chip circuits, etc. It is needless to say thatreference numerals 10 and 11 may respectively be configured as thesource and drain in FIG. 1 and other drawings.

The charge storage region is not limited to the constitution thereof bya silicon nitride film. As the charge storage region, a conductivefloating gate electrode (e.g., polysilicon electrode) covered with aninsulating film, or a conductive particle layer covered with aninsulating film, or the like, may be adopted. The conductive particlelayer can be made of, for example, nanodots which constitute polysiliconin dot form.

A semiconductor integrated circuit device according to the presentinvention is not limited to a data processor like a microcomputer andcan be widely applied even to a system LSI which is relatively large inlogic scale, and which has been system-on-chipped, etc.

Advantageous effects obtained by a typical or representative one of theaspects of the invention disclosed in the present application will bedescribed in brief as follows:

Memory information can be read at high speed from a nonvolatile memorycell transistor formed in a semiconductor integrated circuit device.

A parasitic resistance value of a channel portion of the nonvolatilememory cell transistor formed in the semiconductor integrated circuitdevice can be reduced.

It is possible to prevent charges of one polarity from being constantlytrapped into the nonvolatile memory cell transistor formed in thesemiconductor integrated circuit device.

It is possible to suppress deterioration of data retentioncharacteristics due to undesired leakage of charges stored in thenonvolatile memory cell transistor formed in the semiconductorintegrated circuit device.

A high voltage MOS transistor, which impairs a quick response and islarge in thickness, can be eliminated from a signal path for readingmemory information from the nonvolatile memory cell transistor formed inthe semiconductor integrated circuit device.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor substrate having a main surface; a first gate electrodeformed over the main surface of the semiconductor substrate; a firstinsulating film formed between the first gate electrode and thesemiconductor substrate; a second gate electrode formed over the mainsurface of the semiconductor substrate; a second insulating film formedbetween the second gate electrode and semiconductor substrate, thesecond gate electrode being disposed adjacent to the first gateelectrode; a third insulating film formed between the first and secondgate electrodes; a semiconductor region formed in the main surface ofthe semiconductor substrate and positioned on a side of the first gateelectrode; a second semiconductor region formed in the main surface ofthe semiconductor substrate and positioned on a side of the second gateelectrode; a first metal silicide layer formed on a top surface of thefirst gate electrode; and a second metal silicide layer formed on a topsurface of the second gate electrode, wherein the first insulating filmincludes a charge accumulation layer, wherein the first metal silicidelayer on the first gate electrode and the second metal silicide layer onthe second gate electrode are spaced by the third insulating film, andwherein a highest point of the first silicide layer from the mainsurface of the semiconductor substrate is different from a highest pointof the second silicide layer from the main surface of the semiconductorsubstrate.
 2. The semiconductor device according to the claim 1, whereinthe highest point of the first silicide layer is higher than the highestpoint of the second silicide layer.
 3. The semiconductor deviceaccording to the claim 1, wherein the first insulating layer iscomprised of a silicon oxide film and a silicon nitride film.
 4. Thesemiconductor device according to the claim 1, wherein the firstinsulating film has a first thickness, wherein the third insulating filmhaving a second thickness, and wherein the second thickness is greaterthan the first thickness.
 5. The semiconductor device according to theclaim 1, wherein a metal silicide film is formed on each of the firstand second semiconductor regions.
 6. The semiconductor device accordingto the claim 1, wherein the third insulating film comprises a siliconoxide film and a silicon nitride film.
 7. The semiconductor deviceaccording to the claim 1, wherein the charge accumulation layercomprises a silicon nitride film.
 8. The semiconductor device accordingto the claim 1, wherein the second gate electrode is disposed at asidewall surface of the first gate electrode.
 9. The semiconductordevice according to the claim 1, wherein the metal silicide film on thefirst gate electrode and the metal silicide film on the second gateelectrode are formed of the same film.